refactor: drop simplejson requirement

After much benchmarking, it was decided that it is preferable to
remove the simplejson dependency. This comes with the caveat that json
handling is slower on Python<=2.6, but faster on Python>2.7, including
Python 3 and pypy.

The recommendation moving forward is to deploy Marconi on Python>2.7.

Change-Id: If7a9380bd32023ec973fd2f267255723aaa49306
Merge "refactor: drop simplejson requirement"