travis: Build qemu-arm with MP_ENDIANNESS_BIG=1 to test bigendian build.
travis: Build qemu-arm with MP_ENDIANNESS_BIG=1 to test bigendian build.

Eventually it would be good to run the full test suite on a big-endian
system, but for now this will help to catch build errors with the
big-endian configuration.

Change-Id: Id19f4e3ad96e00009f8ed5ac85f6d995879dcdc2
Signed-off-by: Damien George <99e6b749acbfbe2a596df99e91d24d6e1fdbee00@micropython.org>

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython