Mention that debugging can be slow.

https://github.com/rails/rails/pull/3180\#issuecomment-14705821 has
mention of rendering going from 1-2ms to 4 seconds with this on,
which seems reasonable: debugging is slow.
Make sure that the test case is run under test env

* Unset $RAILS_ENV that got set by abstract_unit to trigger the default.
* split out environment setting since Ruby 1.9.3 doesn't support inline
  ENV setting.
Merge pull request #9666 from sikachu/master-fix-test_test_failure

Make sure that the test case is run under test env
Use platforms instead of conditionals in Gemfile
Move benchmark-ips to test group
There's no need to install test group in travis
Skip fetching path if the iteration is going to be skipped
Replace assert_(no_)match args from regexps to strings to remove warnings

Using regexps as arguments without parentheses end up raising:

    warning: ambiguous first argument; put parentheses or even spaces
Show warning message if delete_all is firing callbacks

`post.comments.delete_all` will fire callbacks if :dependent option is
:destroy . It will be fixed in Rails 4.1 . In the meantime display
a warning . Look at #9567 for details .
Merge pull request #9668 from neerajdotname/9567-delete-all-warning

Show warning message if delete_all is firing callbacks