Improve and stabilize local and Travis tests

A few things:

* Cleaned up unused development gems.
* Added pre `binding_of_caller` for JRuby tests, as they currently error out.
* Silenced a deprecation warning on Rails 4.2 stable.
* Got more strict tests by not allowing 4.1 and 4.2 stable to fail.
* Fixed a transient failure caused by blocking wrong networks.
* Fixed a mistakenly duplicated test.

JRuby and Rubinius tests are still failing because of non-portable exception formatting.
I'll address this in a separate PR.

The transient failures were happening, because sometimes, whitelisted IPs were set to
the private networks of `192.168.0.0/16` and `172.16.0.0/12` during testing, which is a
problem by its own.

Fixing this now and I'll investigate why this is happening and whether
there is a problem with the initialization code.