Switch test suite to RSpec

Removes Bacon and Mocha

Reasoning explained here: https://github.com/pry/pry/issues/277#issuecomment-51708712

Mostly this went smoothly. There were a few errors that I fixed along the way, e.g. tests that were failing but for various reasons still passed. Should have documented them, but didn't think about it until very near the end. But generaly, I remember 2 reasons this would happen: `lambda { raise "omg" }.should.raise(RuntimeError, /not-omg/)` will pass because the second argument is ignored by Bacon. And `1.should == 2` will return false instead of raising an error when it is not in an it block (e.g. if stuck in a describe block, that would just return false)

The only one that I felt unsure about was spec/helpers/table_spec.rb `Pry::Helpers.tablify_or_one_line('head', %w(ing)).should == 'head: ing'` This is wrong, but was not failing because it was in a describe block instead of an it block.  In reality, it returns `"head: ing\n"`, I updated the test to reflect this, though I don't know for sure
this is the right thing to do.

* This will fail on master until https://github.com/pry/pry/pull/1281 is merged.
* This makes https://github.com/pry/pry/pull/1278 unnecessary.