Larger Travis build matrix

[smmaurer]

This PR expands the Travis build matrix:

1. Python 2.7 + Pandana 0.3
2. Python 2.7 + Pandana 0.4
3. Python 3.6 + Pandana 0.3
4. Python 3.6 + Pandana 0.4

Combination 4 fails because Travis hangs in the demo notebook. We haven't been able to diagnose this or reproduce it outside of Travis.

Combination 3 fails at a later point, because of a temporary incompatibility between the latest versions of matplotlib and basemap.

These two builds are "allowed" to fail, so that the overall job passes.

We can merge this if it seems helpful, or drop it!

[smmaurer]

Updates:

1. Matplotlib 3.0.2 fixes one of the problems that was causing Python 3 Travis builds to fail -- combination 3 in the original comment now works.

2. Combination 4 still causes Travis to hang, but works everywhere else.

I've edited the Travis script to allow only combination 4 to fail, and also to express the version constraints differently now that there's an 0.4.1 release of Pandana.

[sablanchard]

looks good!