try using wheels.astropy.org with pip
try without index.html
added wheels2 to list of links
Removed "-q" in pip calls so that error message go through
Shanged pytest-xdist to use the same pip command as others
Added "--use-mirrors" in case source fallback is necessary
removed "-qq" from apt-get calls in .travis.yml
Merge pull request #1365 from eteq/test-wheels-org

Change travis tests to use wheels.astropy.org and remove quiet mode
Allow addition/subtraction of Quantities convertible to times
Also add a rough "to" method for TimeDelta