Install GCC 5 on buildbots to replace GCC 4.9.

LLVM is dropping support for GCC 4.9. This patch adds
a GCC 5 installation to the buildbot image so we can upgrade
the 4.9 bot to GCC 5.

As a temporary workaround until zorg updates, we install GCC 5
as GCC 4.9.

llvm-svn: 355334