-current Steam Fix

I recently ran into an issue after some upgrades where steam would "launch" but all of the actual content screens would be black and never load. Turns out that it's a crash in steamwebhelper: it uses the Steam Runtime libz.so but the system libpng.so, which is compiled against a newer libz. All you need to do to get it running again is to rm ~/.local/share/Steam/ubuntu12_64/steam-runtime-heavy/pinned_libs_64/libz.so.1