add coverage test for travis

For issue #2494

- this is a quick version -  just wanted to get it up so long. Will check through travis docs now and try testing it.

Any feedback welcome. I tried to follow more or less what was done in [nipy's](https://github.com/matthew-brett/nipy/commit/20bf11ff5952abce9e2d8256ccae5a9453e5651c) commit.