Add TravisCI testing env with Mayavi installed (#341)
Add TravisCI testing env with Mayavi installed (#341)

Installing Mayavi can cause other packages to be downgraded
(particularly numpy). This might cause something to break unexpectedly
so it's best to run the tests with Mayavi installed as well. This is an
added build for the TravisCI matrix.
cmeessen/fatiando,drandykass/fatiando,drandykass/fatiando,fatiando/fatiando,fatiando/fatiando,victortxa/fatiando,cmeessen/fatiando,rafaelmds/fatiando,victortxa/fatiando,santis19/fatiando,rafaelmds/fatiando,santis19/fatiando