Update build_http_handlers.read_file to avoid usage of jsonschema.compact.urlopen

[macisamuele]

The objective of this PR is to remove bravado-core dependency on jsonschema.compact.urlopen as it is breaking python 2.7 tests.

The breaking change has been introduced by jsonschema>=3.0.0, specifically on https://github.com/Julian/jsonschema/pull/472/files which alters the behaviour of jsonschema.compact.urlopen on python2.
The alteration consist of wrapping the result of urlopen into a closing which leads to closing an object that does not have close attribute and consequently making the tests failures.

Starting from bravado-core>=5.10.0 we should be supporting jsonschema>=3.0.0 but this specific change was not announced on the CHANGELOG even if it was changing a public interface

Once this PR is approved we should be releasing a new version making explicit the fact that this is the first version that properly works with jsonschema>=3.0.0

[coveralls]

Coverage decreased (-0.002%) to 98.456% when pulling 441265f on macisamuele:maci-issue-318 into 18e3574 on Yelp:master.