Run Travis CI tests on multiple pandas versions

All tests on Travis CI will now run explicitly on pandas 0.12.0 and pandas master.

The downside is that pull requests may show a failing test due to a pandas change that broke something unrelated.

The benefits are that we will quickly catch pandas changes that will affect things that GeoPandas depends on, and also it will be clearer what will break in pandas release versions if we start using the improved typing and subclassing available in pandas master (see #26).