Travis: Install GNU parallel to speed up tests

Debian (and by extension Ubuntu) systems have two packages that provide
the command `/usr/bin/parallel`: moreutils and parallel.  Unfortunately
they do not have compatible syntax.  stb-tester depends on moreutils so
we have to install that package.  With GNU parallel (provided by the
Debian package "parallel") we can parallelize the tests we run and so
make them faster.

With this commit we build and install GNU parallel as a part of Travis
commands to allow us to use GNU parallel on Travis' Ubuntu 12.04 and
thus accelerate the testing process.