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
How about an if/then here, to echo "build failed" or something a bit more obvious to non-bashy folks, and a "build passed" in the event of a success ?

Also, if we're doing errors-only, should we capture warnings and print them when giving the final status? Might be useful to know the number of warnings, even if we don't fail based on them.