JRuby does not support setting/unsetting environment variables.
Thus, we will launch an entirely new process without the parents env when
we run `bundle install` inside the generated rails apps, forcing them to
use their own Gemfile, instead of the BUNDLE_GEMFILE setting from the main
gem codebase.
Previously, the railties required by the project gemspec was in conflict with
the rails versions being pulled in by Appraisal.
There were situations where a newer rails app was generated by `rails new...`
even though the Appraisal gemfiles/ value should have been setting it correctly.
We need to be able to use the versions of things specified in the appraisal gemfiles/
specify the version of rails for the initial generation of the applications,
but subsequently allow those applications to point to the `Gemfile` within
the newly generated rails apps, rather than wherever BUNDLE_GEMFILE was previously
set by Appraisal.
