Force pre-commit run for python 2.7 tests

[macisamuele]

With this PR we'll enforce the run of all the pre-commit hooks during the execution of Python 2.7 tests.
Doing so we'll enforce a prettier code base 😃

Example of travis logs:

• python 2.7 -> runs pytest and pre-commit (https://travis-ci.org/macisamuele/bravado-core/jobs/208207394)
• python 3.5 -> runs pytest only (https://travis-ci.org/macisamuele/bravado-core/jobs/208207395)

[coveralls]

Coverage remained the same at 97.587% when pulling 9423a2f on macisamuele:force-pre-commit-hooks-run into 3333c72 on Yelp:master.