Added GML support

[meyer1994]

Fixes #11

When trying to use GML drivers it resulted in errors. Using gdal in python, ogr.Open returned None. When trying to read files using ogrinfo, it resulted in this:

$ ogrinfo out.gml 
ERROR 1: Unable to create Xerces C++ or Expat based GML reader, Xerces or Expat support not configured into GDAL/OGR.
ERROR 1: File out.gml appears to be GML but the GML reader can't
be instantiated, likely because Xerces or Expat support was
not configured in.
FAILURE:
Unable to open datasource `out.gml' with the following drivers.
  -> PDS4
  -> JP2OpenJPEG
  -> PDF
  -> MBTiles
  -> EEDA
  -> ESRI Shapefile
...

The addition of these libraries did not result in big changes in layer size when generating it. It stayed in the range of ~34Mb.

[vincentsarago]

excellent, thanks a lot @meyer1994

could you add ogrinfo --formats | grep "GML" in https://github.com/RemotePixel/amazonlinux/blob/master/scripts/tests.sh

Note to myself: I need to fix some stuff in the circleci config to enable test to run for PR

[meyer1994]

Test added.