Added cfchecker to travis-ci

This PR modifies ```.travis.cml``` so that cfchecker (and its dependency cdat-lite) get installed via pip. It wraps the resulting ```cfchecks``` commandline utility in a script called cfchecker (providing the required args to point to the cf standard name and area types xml files (downloaded) and the udunits xml file) so that the existing codebase can call it.