Get tests passing with Oscar 0.7

This required installing gdal as there was a django.contrib.postgis
regex error trying to determine the 'linear units'.

I also needed to change the syntax used for specifying points when
creating stores in tests. Not exactly sure why but I've lost too many
hours of my life to debugging geodjango/PostGIS issues - I'm not keen to
lose more.
Pin requirements

And pin six==1.5.2 to handle a silly versioning conflict caused by
django-extra-views which forces this version.
Drop support for Oscar 0.5

Which means we no longer need oscar_testsupport.