travis: Install power8 simulator for make check

This downloads and installs mambo, the power8 simulator, including some
dependencies to ensure it runs on Travis' ancient Ubuntu 12.04.

Without the newer libc, you'll see a warning like this:

 /opt/ibm/systemsim-p8/bin/systemsim-pegasus:
  /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.20' not
  found (required by /opt/ibm/systemsim-p8/bin/systemsim-pegasus)

Where GLIBCXX_3.4.20 is GCC 4.9. This is why we require libstdc++ from the
ppa.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>