Avoid jsonschema 1.4.0 - Release was pulled.

JSONSchema 1.4.0 appears to have been an accidental release of 2.0
code under the 1.4.0 version. It's been pulled from PyPi, but still
exists on the OpenStack mirror.

Discussion with the infra folks suggests updating the requirements
file rather than removing the release from the mirror is the best
solution.

Change-Id: I7619cc41c7924f2f02c8cf8d17034e58ac3b116c
Merge "Avoid jsonschema 1.4.0 - Release was pulled."