Relax simplejson dependency to >= 3.0.0

Python3 support was added to simplejson in 3.0.0, so any version after
that should be fine.  Strictly requiring 3.3.3 can introduce version
conflicts with other libraries.
Merge pull request #29 from thobbs/relax-simplejson-dependency

Relax simplejson dependency to >= 3.0.0