Work around 2.6 relpath bug that was breaking a test.

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

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>

Conflicts:
	lib/pyld/jsonld.py
tests: compatibility with Python 2.6
travis-ci: Python 2.6 build enabled