fix(build_library): Re-enable zeroing free-space but in a sane way.

As of Linux 3.2 loopback supports discard by punching holes in the
underlying file. This doesn't actually seem to impact things right now
since we are writing to fresh filesystems but might as well do this to
prevent wasted space from sneaking in later on.
Merge pull request #66 from marineam/fstrim

fix(build_library): Re-enable zeroing free-space but in a sane way.