Fixes the ScalarMapContainer/MessageMapContainer implementations on
Python 3.4, by dynamically allocating their PyTypeObjects using
PyType_FromSpecWithBases, instead of statically allocating them. This is
necessary because Python 3.4+ disallows statically allocating a class
with a dynamically allocated parent.
Signed-off-by: Dan O'Reilly <oreilldf@gmail.com>

Signed-off-by: Dan O'Reilly <oreilldf@gmail.com>

Signed-off-by: Dan O'Reilly <oreilldf@gmail.com>

Signed-off-by: Dan O'Reilly <oreilldf@gmail.com>

Signed-off-by: Dan O'Reilly <oreilldf@gmail.com>

The tests are run from Travis in the same way as on other
platforms. Currently some expected failures - but only expected
in that they're what we got to start with. Will try to fix them in
other pull requests.

This involved fixing a few important bugs in the
Ruby implementation -- mostly cases of mixing
upb field types and descriptor types (upb field
types do not distinguish between int/sint/fixed/sfixed
like descriptor types do).

Also added protobuf-specific exceptions so parse
errors can be caught specifically.

Change-Id: Ib49d3db976900b2c6f3455c8b88af52cfb86e036

Remove the OS X checks from travis.sh, instead use excludes to not fire up a vm for something we can't test.

Fixes #333.