Change 'lib' to 'mrblib' in mruby installation

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.
Merge pull request #1502 from rjsamson/master

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