Use stub_const in favor of old, custom patch

MSP-11399

With the introduction of stub_const in rspec-mocks 2.11, there was no
need for `with_constants` since rspec can handle the constant stubbing
itself now.  New code used stub_const inside the with_constants block,
but no one went back yet to refactor the old code to use stub_const.
Merge pull request #216 from rapid7/feature/MSP-11399/stub_const

Use stub_const in favor of old, custom patch