Switch to gcc 4.9 on travis

From 4.8 gcc branch, ppa:ubuntu-toolchain-r/test only has 4.8.1, but we
can't use it as it has bug in libstdc++ which prevents building
libSDL2pp. 4.8.2 or 4.8.3 would probably be fine, but since I don't know
PPA to get them from, just switch to gcc 4.9 for now