[Fix tests] Remove spec=Draft4Validator from mock_validator

[macisamuele]

Tests are currently failing on master

• last green build: https://travis-ci.org/Yelp/bravado-core/jobs/646621656
• last build: https://travis-ci.org/Yelp/bravado-core/jobs/646621656

The difference in the two builds is limited to the mock library:

• mock==3.0.5 in the green build
• mock==4.0.0 in the failing build

The following code is causing the test setup to fail. The change was introduced by https://github.com/testing-cabal/mock/pull/476/ as part of backporting the "official" unittest.mock back into the package.

In order to fix the test we could:

1. modify jsonschema, which does raise the exception in purpose (deprecation related task)
2. not define the spec while defining the mock
3. limit mock to mock<4

I'm planning for approach 2 as it does not reduce our confidence around the whole interaction (especially as we have some integration tests) and does not require a lot of effort.

[coveralls]

Coverage remained the same at 98.502% when pulling 8004f5b105ff6182cb41e593b85b5e457e37afad on macisamuele:maci-fix-tests into 9c20a251f016870f1f716b7217d3dae4ebbf9c56 on Yelp:master.