Clytwynec/add travis configuration

@benpatterson Please review. 

For now, this runs only the html builds (no latexpdf).  I've made it so that builds will fail if there are sphinx warnings during the build, even if the build itself doesn't error out.  @mhoeber Is that the expected behavior?

Most recent build: https://travis-ci.org/clytwynec/edx-documentation/builds/43651494
Commented on this below, but this might be a good place to echo the number of warnings.