Suggest 'ridk install' when MSYS2 could not be found.
Suggest 'ridk install' when MSYS2 could not be found.

Fixes #22

larskanis/rubyinstaller2,oneclick/rubyinstaller2,oneclick/rubyinstaller2,larskanis/rubyinstaller2,larskanis/rubyinstaller2,oneclick/rubyinstaller2