accept options to gem install

Background: Our build broke recently due to compass 1.0.1 coming out and changing a behavior of `@extend`. At this point we realized we didn't have the compass version locked down.  Specifying `"compass:0.12.7"` wasn't enough because another gem we installed (e.g., `compass-css-arrow`) was still pulling in compass `1.0.1`. The only way I could find to actually lock down our compass version was to pass `--ignore-dependencies` to the install command for the other gems.

So this PR adds an escape hatch to pass options to `gem install`. That lets us specify `--ignore-dependencies` and lock the version down.