x8d4x:~...$ rvm --default use 2.1.5
Using /Users/.../.rvm/gems/ruby-2.1.5
x8d4x:~ ...$ rvm ls

rvm rubies

   ruby-1.9.3-p547 [ x86_64 ]
   ruby-2.1.2 [ missing bin/ruby ]
   ruby-2.1.3 [ x86_64 ]
=* ruby-2.1.5 [ x86_64 ]

# => - current
# =* - current && default
#  * - default

x8d4x:~ ...$ rvm uninstall 2.1.3
ruby-2.1.3 - #removing rubies/ruby-2.1.3.......................................................................................................................................................................................................................................|
Error running '__rvm_rm_rf /Users/.../.rvm/rubies/ruby-2.1.3',
showing last 15 lines of /Users/.../.rvm/log/1416771813_ruby-2.1.3/remove.rubies.log
rm: /Users/.../.rvm/rubies/ruby-2.1.3/lib/ruby/gems/2.1.0/gems/tzinfo-1.2.2/test/zoneinfo: Permission denied
rm: /Users/.../.rvm/rubies/ruby-2.1.3/lib/ruby/gems/2.1.0/gems/tzinfo-1.2.2/test: Permission denied
rm: /Users/.../.rvm/rubies/ruby-2.1.3/lib/ruby/gems/2.1.0/gems/tzinfo-1.2.2/tzinfo.gemspec: Permission denied
rm: /Users/.../.rvm/rubies/ruby-2.1.3/lib/ruby/gems/2.1.0/gems/tzinfo-1.2.2: Directory not empty
rm: /Users/.../.rvm/rubies/ruby-2.1.3/lib/ruby/gems/2.1.0/gems: Directory not empty
rm: /Users/.../.rvm/rubies/ruby-2.1.3/lib/ruby/gems/2.1.0: Directory not empty
rm: /Users/.../.rvm/rubies/ruby-2.1.3/lib/ruby/gems: Directory not empty
rm: /Users/.../.rvm/rubies/ruby-2.1.3/lib/ruby: Directory not empty
rm: /Users/.../.rvm/rubies/ruby-2.1.3/lib: Directory not empty
rm: /Users/.../.rvm/rubies/ruby-2.1.3: Directory not empty
++ typeset ret=1
++ rvm_debug '__rvm_rm_rf error removing target dir '\''/Users/.../.rvm/rubies/ruby-2.1.3'\''.'
++ ((  0  ))
++ return 0
++ return 1