`bundle exec rake` now works in addition to `rake`

The new VM update to Travis upgraded RVM such that all commands are
surreptitiously surrounded in a `bundle exec`, which will require Bundler. Doing
so will set RUBYOPT to '-r bundler/setup' which will cause Bundler to get
autorequired when descending into subshells, such as the ones we use to house
test suites. We ourselves require Bundler within a test suite, but because
Bundler has already been loaded our require takes no effect and so the test
suite doesn't run correctly. This commit fixes this to unset RUBYOPT before
spawning a subshell.