Add missing imports

[abretaud]

A little fix for an exception raised when calling run_tool for example:

Traceback (most recent call last):
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/parsec/decorators.py", line 13, in custom_exception
    return wrapped(*args, **kwargs)
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/parsec/decorators.py", line 37, in dict_output
    output = wrapped(*args, **kwargs)
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/parsec/commands/tools/run_tool.py", line 20, in cli
    return ctx.gi.tools.run_tool(history_id, tool_id, json_loads(tool_inputs))
NameError: global name 'json_loads' is not defined

Seen in https://travis-ci.org/bgruening/docker-galaxy-stable/jobs/467400897 It would be cool to push a new release on pypi to fix it for https://github.com/bgruening/docker-galaxy-stable/pull/464 (though there are a few other errors there)

[hexylena]

Sure, it auto-publishes. If you push to master updating the version numbers in all the places + add a tag, it'll automatically be released on pypi.

[hexylena]

Done

[abretaud]

Cool, thanks!