paul@paul-ThinkPad-T430:~$ sudo gem install knife-esx
[sudo] password for paul:
ERROR:  Error installing knife-esx:
	esx requires rbvmomi (>= 0)

ERROR:  Error installing rbvmomi:
	ERROR: Failed to build gem native extension.

    /usr/bin/ruby2.1 extconf.rb
mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h

extconf failed, exit code 1

Gem files will remain installed in /var/lib/gems/2.1.0/gems/nokogiri-1.6.8.rc2 for inspection.
Results logged to /var/lib/gems/2.1.0/extensions/x86_64-linux/2.1.0/nokogiri-1.6.8.rc2/gem_make.out