Fix mruby install - change mruby 'lib' directory to 'mrblib'

In mruby the 'lib' directory is called 'mrblib'. Right now `rvm install mruby` errors out on the trying to cp the lib directory: `.rvm/src/mruby-head/lib: No such file or directory`.
This commit changes 'lib' to 'mrblib' to address this issue.

Tested this locally and everything works as it should.