allow people to set a local .Gemfile so that things like ruby-debug are not required for regular development

As rake 0.9.2.2 is out with the same 
fix as 0.9.3.beta having

THis version is compatible with 3.1 and above

Don't install ruby-debug if running the test suite on Travis,
linecache19 is the main offender, very very slow.

And do not install pg if Travis is bundling the gems, pg will be setup
on Travis soon.

This reverts commit 1028226b.

Reason: No code changes can be done in docrails.

Removing require_relative gem. Linecache new version 0.46 is out and it's loading rbx-require-relative > 0.0.4

Add Gemfile dependency for require_relative on mri18. This is a dependency of linecache which is a dependency of ruby-debug. However, the latest release of linecache (0.45) omits the dependency from its gemspec, causing the build to fail. I have emailed the maintainer so we should remove this when it's fixed upstream.