travis: Install libstdc++ from ppa for systemsim

/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)

GLIBCXX_3.4.20 is gcc-4.9

Signed-off-by: Joel Stanley <joel@jms.id.au>