pyld: relpath fix for Python 2.6

* relpath('/foo', '/') returns '../foo' instead of just '/foo' on 2.6. This
  was causing the relative URL round-tripping test to fail.

Reported-by: Yoan Blanc <yoan.blanc@cern.ch>
Acked-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>

Conflicts:
	lib/pyld/jsonld.py
tests: compatibility with Python 2.6

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
Acked-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>
travis-ci: Python 2.6 build

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
Reviewed-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>