Remove simplejson dependency for Python>=2.7

[laurentb]

Even though the json module is in Python 2.6, OrderedDict is only present in 2.7.

[zacharyvoase]

One question: since we're installing simplejson in the install_requires anyway (for all Python versions), is there an advantage to using the stdlib versions when they are present?

[laurentb]

When using pip to install the package, not really, unless there is a way to have a conditional dependency. However I've seen and wrote Gentoo ebuilds with that logic.

Also, for Python 3 and onwards, simplejson is only available from the stdlib.