Add whiny_nils configuration variable to control behavior on nil bindings

[fancyremarker]

The use case is that sometimes I want to be able to call garner.bind(something) { } where something may be nil. The result won't be cached, but it also won't raise an exception. I set the configuration variable to default to the old behavior.

[fancyremarker]

Oh, also pinned rspec and mongoid in the development dependencies because Garner doesn't work with RSpec 3.x or Mongoid 4.x.

[dblock]

Garner works fine with Mongoid 4. I'll take a look at the specs, https://github.com/artsy/garner/issues/71.

[dblock]

You need to document this in the README, too.

[fancyremarker]

Rebased and addressed the following issues:

• whiny_nils? can be used along with whiny_nils as a config accessor (using it as a setter won't work because of Ruby syntax rules).
• Added a Garner::Cache::NilBinding exception. I agree that's much more helpful than a generic NoMethodError.
• Fixed CHANGELOG and added README note.

Did not address the Mongoid 3.x development dependency issue since we can do that on #71.

[dblock]

Perfect, merging.