Remove Rails warning messages in specs

* Set `config.secret_key_base`
* Remove `config.whiny_nils` because it doesn't exist anymore.
Turn off Travis emails
Remove empty Rake task file
Exit if bundler is not installed
Make README friendlier

* Remove repetitive language
* Explicitly refer to `Griddler::Email`, instead of referring to "an email"
* Remove "just" (http://css-tricks.com/words-avoid-educational-writing/)
Add testing module for adapter gems

This provides some test helpers that will be used across all adapters.
Fix deprecations
Extract shared_examples_for Griddler adapters