Revert "Merge pull request #386 from tomas-abrahamsson/gpb-recompilation-detection"

This reverts commit 81063d33f0939416f9bc0a7fc6a4cc326a0059d2, reversing
changes made to 6584defdda209d43505baee21b0f9f4d7f829573.

This was reverted because it changed the gpb compiler to
not use rebar_base_compiler.  That change was to add support for
target name prefixes, but it is better to add that support to
rebar_base_compiler.
Merge pull request #413 from tomas-abrahamsson/undo-386-merge

Revert "Merge pull request #386 ... gpb-recompilation...detection"
Fix #415 (reltool vsn check)

* Refactor reltool version check to use vsn app key if ebin/reltool.app
  exists.

  OTP releases prior to R14 didn't install the app file, so we still
  have to support getting the version string from the lib_dir path. If
  you're using an R13B03 or R13B04 install that doesn't have version
  strings in lib_dir paths (lib/reltool vs lib/reltool-0.5.3), we won't
  be able to retrieve the version string. Given R13's age and that it's
  unusual to have vsn-less lib_dir paths, this shouldn't be problem.

* While at it, delete trailing white space.