Install alex and happy in the sandbox
Set upper bound for `html-conduit`

1.1.1 has a bug that means we can't use it, and we can't jump above (to
1.1.1.1) because of other dependency requirements.
Make travis builds functional again

- Allow any 7.8 GHC version

Seems the exact version, 7.8.2, is no longer present.

- Use dummy OAuth keys when testing

The existing secure variables were tied to the old private repository job.
They're not really needed anyway: as long as the values are non-empty, the tests
will run.