.travis.yml: split VM based builds

The Trusty based builds run a little slower than the main container
based ones. This is also true for the latest version of Clang. The
builds are getting very close (and occasionally run over) the 50 minute
timeout. Rather than partitioning by target I just split them into
linux-user and system builds.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>