Check SASS compiles as part of Jenkins test runner

* Running asset precompile will catch SASS syntactic/reference errors earlier.
* Use set -e, to immediately fail if any command fails,  rather than than relying
on a manual exit, as we're running more than one 'test' command, more robust.
Merge pull request #616 from alphagov/test-asset-precompile

Check SASS compiles as part of Jenkins test runner