Fix cache-related tests

The tests were failing because the StoryIndex signal handlers were
updating the cache value (correctly) before the assertion check.  Instead
of checking that the cache value is None, just check that it's not the
stale dummy value.

Closes #563
New mirror for downloading Solr in CI build

The mirror used for downloading a more recent version of Solr as part of
the Travis CI build didn't work anymore.  Update to a working mirror.

Also, remove redundant command to install the solr-jetty package.  It was
being run in both a fabric task and in the ci_before_install.sh script.

Addresses #564
Crop thumbnail images as well as scaling them

Images are cropped from the center of the image horizontally and from the
top edge vertically.

Addresses #276
Use CSS to crop story image thumbnails in explorer

Addresses #276.
Disable featured image cropping

For now, try using CSS cropping instead of actually resizing the image.

Addresses #276