Support Postgres versions 9.1, 9.2 & 9.3 in travis

Hi, 

I added the before install block from plv8js to the travis file, which adds Postgres versions 9.1, 9.2 & 9.3 to the travis testing matrix. 

Note that 9.0 fails but I wasn't sure whether that's too old to matter so I just removed it for the time being.