Fix JSON error in Coffeescript sourcemap transformation

Fixes intermittent JSON error when transforming coffeescript sourcemaps:

...
  File "civet/compilers/coffeescript.py", line 80, in compile
    map_data = json.load(f)
  File "/usr/lib/python2.7/json/__init__.py", line 290, in load
    **kw)
  File "/usr/lib/python2.7/json/__init__.py", line 338, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python2.7/json/decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python2.7/json/decoder.py", line 384, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded

Add ES6 Support

Adds ECMAScript 6 support via babel.

Note this release is a significant refactor and includes some breaking API changes:

• precompile_coffee_and_sass_assets() was renamed to precompile_assets().

(Note 2.0.0 was never released due to a distribution error.)