Fixed Travis build errors with Python version conflicts.
Fixed Travis build errors with Python version conflicts.

Merge pull request #633 from landlab/mcflugen/fix-build-conflict-errors
cmshobe/landlab,landlab/landlab,amandersillinois/landlab,cmshobe/landlab,amandersillinois/landlab,cmshobe/landlab,landlab/landlab,landlab/landlab