CLC-4267 - Make sure travis build fails when scss-lint fails

https://jira.ets.berkeley.edu/jira/browse/CLC-4267

Big change was running it instead of "before_install" into "script"
Example of failure at https://travis-ci.org/ets-berkeley-edu/calcentral/jobs/37350486
@ctweney @raydavis this should improve our build time.
`--jobs` allows it to run in parallel
`cache` should allow us to cache the bundle installations