Fix up webhook specs
Fix up webhook specs

This is a rather large commit, and merits some extended explanation.

1. Upgrading to RSpec 3 exposed some problems with the existing specs.
   More precisely, some specs were failing but these failures were
   never reported. See 
https://github.com/rspec/rspec-core/issues/2569#issuecomment-427651669
   for more explanation.
2. While reviewing, I came to the conclusion that the failing specs
   (ones that compare keys in the payload) are not essential, so I
   removed them
3. Fixed order-dependent specs, which set signing_private_key but did
   not revert it.
4. Removed unnecessary methods.

travis-ci/travis-tasks,travis-ci/travis-tasks