Add Travis tests

Here is how sympy is tested in Sage: https://groups.google.com/d/msg/sympy/y7zZNlAs7Uc/qfiPNAyqPJ8J, so we should do the same. For now I would just test this repository only, i.e. one has to manually update the submodule in a PR, and if tests fail, one has to first fix it in symengine.

Perhaps later we can also test this for every commit in symengine itself. 
