Travis-CI Integration

hello lovely digitalbazaar folks!

attached find some changes that start configuration for travis-ci for pyld... it looks like 2.5 support is kind of a non-starter at this point (even tried some __future__ shenanigans, just to get it installing), 2.6 doesn't like cmp_to_key (and may not be well supported in 3.x as of some point), and 3.x is not into `print` (but we knew that). I'd take a whack at those if you'd like, and maybe tune up the test output to be more travis-friendly.

it seems like the reference implementations of all the json-ld stuff should probably be tested a) individually, as they are developed and b) as a group when the spec changes... this kind of confidence is important for broader adoption, and will help highlight interoperability challenges! i am most comfortable in the python and javascript environments, and would happily add more testing capabilities in these places.

hope this helps!