A few improvements to the mocks/expectations part of the build.

- Our `bundle install` was installing core's bundle, even for mocks
  or expectations, because travis has exported the BUNDLE_GEMFILE
  env var. We need to unset it so that the correct bundle is installed.
- Clone the repos at a lower depth for faster cloning.
- Use `bin/rspec` just like our normal builds do. Before this wouldn't
  work because of the BUNDLE_GEMFILE issue.
- No need to delete the dir after the build.
Merge pull request #1256 from rspec/fix-travis-build

A few improvements to the mocks/expectations part of the build.