what I did was

git clone git://github.com/francois/piston.git
cd piston
rake install_gem


this dumped a bunch of output, as it built the gem and then asked me for sudo password to install:

...
  File: piston-1.9.3.gem
sudo gem install --local pkg/piston-1.9.3.gem

this succeeded-ie, gem list has piston 1.9.3 in it now.  However, running piston breaks:

bash-3.2$ piston
/opt/local/lib/ruby/site_ruby/1.8/rubygems.rb:142:in `activate': can't activate piston (= 1.4.0, runtime), already activated piston-1.9.3 (Gem::Exception)
        from /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
        from /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.0.991/lib/active_support/dependencies.rb:508:in `require'
        from /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.0.991/lib/active_support/dependencies.rb:353:in `new_constants_in'
        from /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.0.991/lib/active_support/dependencies.rb:508:in `require'
        from /opt/local/lib/ruby/gems/1.8/gems/piston-1.9.3/lib/piston/commands.rb:5
        from /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `gem_original_require'
        from /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `require'
        from /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.0.991/lib/active_support/dependencies.rb:508:in `require'
        from /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.0.991/lib/active_support/dependencies.rb:353:in `new_constants_in'
        from /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.0.991/lib/active_support/dependencies.rb:508:in `require'
        from /opt/local/lib/ruby/gems/1.8/gems/piston-1.9.3/lib/piston/cli.rb:5
        from /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `gem_original_require'
        from /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `require'
        from /opt/local/lib/ruby/gems/1.8/gems/piston-1.9.3/bin/piston:5
        from /opt/local/bin/piston:19:in `load'
        from /opt/local/bin/piston:19


on uninstalling piston and re-installing 1.9.3, I now get the following new error when running piston:

bash-3.2$ piston version
/opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `gem_original_require': no such file to load -- piston/commands/convert (MissingSourceFile)
        from /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `require'
        from /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.0.991/lib/active_support/dependencies.rb:508:in `require'
        from /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.0.991/lib/active_support/dependencies.rb:353:in `new_constants_in'
        from /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.0.991/lib/active_support/dependencies.rb:508:in `require'
        from /opt/local/lib/ruby/gems/1.8/gems/piston-1.9.3/lib/piston/commands.rb:5
        from /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `gem_original_require'
        from /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `require'
        from /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.0.991/lib/active_support/dependencies.rb:508:in `require'
        from /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.0.991/lib/active_support/dependencies.rb:353:in `new_constants_in'
        from /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.0.991/lib/active_support/dependencies.rb:508:in `require'
        from /opt/local/lib/ruby/gems/1.8/gems/piston-1.9.3/lib/piston/cli.rb:5
        from /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `gem_original_require'
        from /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `require'
        from /opt/local/lib/ruby/gems/1.8/gems/piston-1.9.3/bin/piston:5
        from /opt/local/bin/piston:19:in `load'
        from /opt/local/bin/piston:19