[travis] Remove installation of python-dev

The Python development files appear to be already installed on Travis so
remove the apt-get call that tries to pull them in.
[checking] Catch sparse matrix comparison error

The sparse matrix error changed and and NotIMplementedError had to be
causght also to check that also elements of a transition matrix are
non-zero.
[tests] Replace mdptoolbox.utils wth .util

The mdptoolbox.utils module was renamed to mdptoolbox.util and the tests
were still using the old name, Replace all occurances of utils with util