[#60] Drop builds on OSX
[#60] Drop builds on OSX

Python on OSX isn't supported by Travis yet (see
https://github.com/travis-ci/travis-ci/issues/2312). There're a few
workarounds, but it doesn't seem worthy for now.

okfn/datapackage-model-py,sirex/datapackage-py,okfn/datapackage-model-py,okfn/datapackage-py,datapackages/datapackage-py,sirex/datapackage-py,datapackages/datapackage-py,okfn/datapackage-py