Add the option to use a custom default-gems file.

Specially important if this role is part of multiple playbooks to
provision systems with different Ruby gems requirements.

It is recommended that the roles that have this one as a dependency
should have a `files/default-gems` file stored in it. This is for the
admin's own sanity.
Merge pull request #18 from andradei/master

Add the option to use a custom default-gems file.