Make master retry specs more specific

The previous tests were inaccurate, since rspec’s mocking raises an error when expectations are not met: RSpec::Mocks::MockExpectationError

Removed one test that is no longer true (rescuing non-Mysql2 errors)
New config: no_replay_on_master errors/messages

Allow the configuration of the errors and messages that shouldn’t be replayed on master by setting SlavePools.config.no_replay_on_master = {ErrorClass => [‘messages’]}.

This was previously hardcoded.
use .any? in safe_to_replay check
remove deprecation in spec for should_not raise

When using should_not raise_error, a Specific Error class should not be passed in.
Change no_fallback config to use strings.
no_replay_on_master - update readme and comments
Merge pull request #9 from kickstarter/adjust_master_fallbacks

Allow configuration of errors that shouldn't be replayed