Run Travis tests in serial

[djkirkham]

Fix to get the tests passing while we investigate why there are now so many time-outs.

[djkirkham]

Closes #2752

[djkirkham]

The tests do still time out sometimes, but if the --process-timeout value is set too high Travis itself times out. I think this is the best we're gonna get.

@lbdreyer @pp-mo

[lbdreyer]

@ajdawson If you look at the travis output for the python 2.7 job you can see that running it in serial completed without a problem in just under 15 mins

[pp-mo]

What a relief !!!

[ajdawson]

There may be a happy medium somewhere. I've also never seen timeouts locally, but I've never used more than 8 cores for testing. I think Travis nodes present 64 cores which our tests will use. How do things look on 4 cores say?

[lbdreyer]

@djkirkham can you make an issue to remind us to revert this to ensure it is only temporary?

[lbdreyer]

I tested it with different numbers of processors and got the following results (note that I only ran each test once so the timings won't be that reliable):

No. of processors	py27 default tests	py27 min test	py27 coding	py27 example	py27 doctests	py35 default tests	py35 minimum test	py35 coding tests	py35 example tests	py35 doctests
1	15m11	5m41	6m16	8m11	9m22	11m55	4m13	8m10	7m23	10m24
4	6m56	5m29	6m3	6m57	10m8	6m	6m12	6m56	7m4	10m17
8	5m23	5m31	6m32	6m19	6m37	4m26	3m47	6m9	6m34	9m29
12	11m253	5m36	6m29	7m2	10m1	8m22	6m23	6m28	6m53	15m57
16	TimedOut	3m59	4m19	6m40	10m25	TimedOut	5m49	6m47	7m28	9m51

I could do more testing but I think setting 8 processors might be our best bet