Bump Java source and target level to 7

Java 7 was released on July 28 2011.
Java 8 is expected in March 2014.

So, let's stop with ugly hacks like:

  .buckconfig
  [...]
  [buildfile]
    includes = //DEFS

in Buck itself or patches like this[1] in Buck driven projects, and just do
the right thing: upgrade source and target level to 7.

[1] https://gerrit-review.googlesource.com/51830