Allow will_<api_method> to accept a block

  This overrides the default behavior instead of just the default return value
Merge branch 'master' into block_overrides
Move spec from PR to its own describe

Build it out a little more, as well, and hit the other case
Bump version to 0.8.1
Remove compatibility check for rspec < 2.11

Seems reasonable since the gemspec requires ~> 3.0
and its making CI fail:
https://travis-ci.org/JoshCheek/surrogate/jobs/44044307