Revert #default_separator on base test case

Clean teardown methods a bit by relying on the base test case.
Officially drop support to Ruby 1.8/REE and Rails < 3.2
Remove string interpolation backport for Ruby 1.8

Now that 1.8 has been officially dropped, we can remove this extension.
Remove more dead code related to Ruby version compatibility
Remove deprecated :default_exception_handler method
Remove deprecated normalize_translation_keys
Remove deprecated :rescue_format from exception handler
Refactor exception handler is_a? checks
Remove the need for TokyoCabinet and Rufus dependency to run the tests

The key-value store requires a very specific API from the store, and we
can use a simple hash object to match that contract on our tests, so
there's no need for yet another dependency on top of it.
Remove rufus and old AR support from benchmark script

Remove warning: `*' interpreted as argument prefix'`
Remove deprecation of enforce_available_locales and default it to true

Ensure super is called from setup/teardown methods across the test suite
so that the base config/cleanup works as expected.