copy some of astropy's coverage stuff over to travis
.travis syntax error?
added the astropy install command.  Thanks @jwoillez!
Merge pull request #337 from keflavich/travis_for_coverage

Travis for coverage
address @jwoillez's comment: missing lines.  Remove --use-mirrors since
it's deprecated
Merge branch 'travis_for_coverage'
Merge branch 'master' of github.com:astropy/astroquery into issue284-2