Might as well run api test on Travis

[mulkieran]

We have the ability to make the docs, and we do document the code pretty thoroughly. We might want to make sure that we don't lose that ability by making a Travis task to check the docs generation.

[mulkieran]

Actually, sphinx is so tolerant of gibberish in the docs that there is little point in running a test and checking the exit code. Might as well not bother.