"Limit concurrent jobs" to 1 in Travis settings for this repo

[jeremyBanks]

Depending on Travis worker availability, the current configuration could run as many as 8 instances of the test in parallel, one for each version of Python being tested. This can cause Stack Exchange's rate limits be exceeded in some cases, leading to flakey tests. (I think that may be what happened to the last build on master but I'm not sure. I'd like to be able to rule it out, at least.)

It may take longer for the builds to complete, but I think it's probably worth disabling the parallel execution in order to get more trustworthy build results. @Manishearth should consider setting "Limit concurrent jobs" to 1in the Travis interface:

[tripleee]

This is obsolete now, after #165