Summary:
Drop instructions for building on Ubuntu 12.04, 13.10, and 14.04.  The oldest
Ubuntu platform we support these days is 16.04.

Our Travis CI configurations do still test building on Ubuntu 14.04 for now,
but it is not a vanilla 14.04 installation.  We upgrade gcc to 4.9 and CMake to
3.2 in order to build folly on this older Ubuntu version.

Reviewed By: yfeldblum

Differential Revision: D6780815

fbshipit-source-id: 0d27cdbb928d32c1527623c8862d50ce2215dd48