.travis.yml: Add a doc building test step.
.travis.yml: Add a doc building test step.

CellProfiler/python-javabridge,jakirkham/python-javabridge,CellProfiler/python-javabridge,jakirkham/python-javabridge,CellProfiler/python-javabridge,jakirkham/python-javabridge,jakirkham/python-javabridge,CellProfiler/python-javabridge