remote-bzr: fixes for older versions of bzr

Up to v2.0.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
remote-helpers: tests: use python directly

These remote helpers use 'env python', not PYTHON_PATH, so that's where
we should check for the extensions. Otherwise, if 'python' is not
PYTHON_PATH (e.g. /usr/bin/python: Makefile's default), there will be a
mismatch between the python libraries actually accessible to the remote
helpers.

Suggested by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
remote-hg: tests: fix hg merge

Let's specify a merge tool, otherwise mercurial might open one and hang
our tests waiting for user input.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
travis: improve test matrix

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>