Bump Ubuntu version in README

Verified this apt command still works under Ubuntu 14.04 32-bit. Program compiles successfully. Haven't tested 64-bit version, though, so I cannot talk about `libx11-6:i386` `libxfixes3:i386` and `libglu1-mesa:i386`. Yet Ubuntu Package database reports these packages haven't received any upgrades between 13.10 and 14.04 so I wouldn't expect a fail.

* [libx11-6: 2:1.6.1-1ubuntu1 (13.10) -> 2:1.6.2-1ubuntu2 (14.04)](http://packages.ubuntu.com/search?keywords=libx11-6)
* [libxfixes3: 1:5.0.1-1ubuntu1 (13.10) -> 1:5.0.1-1ubuntu1 (14.04)](http://packages.ubuntu.com/search?keywords=libxfixes3)
* [libglu1-mesa: 9.0.0-1 (13.10) -> 9.0.0-2 (14.04)](http://packages.ubuntu.com/search?keywords=libglu1-mesa)