Pd master fixes

The first two of these commits fix the tests against pandas master.
The first removes the GeoSeries override of `__getslice__`. It's handled by `__getitem__`.
The second fixes the call to `read_sql`. Pandas overhauled the SQL interface. The fix here was to use keyword arguments and everything passes down correctly now.

Those fixes work in back to Pandas 0.12.0.

The third fixes a GeoPandas bug where converting the GeoDataFrame to GeoJSON would fail if the geometry column was not named 'geometry'.