Adds --no-wrapper flag for gem installs

We are adding wrapper bin files to the path ourselves, so it doesn't
make sense to have rubygems do it as well.  Also, this avoids conflicts
from happening with duplicate bin file names on install, which requires
user input and keeps homebrew from finishing the install.
Merge pull request #11 from sportngin/no-bin-wrapper

Adds --no-wrapper flag for gem installs