replace shell `patchelf` and `readelf` with ruby equivalents

Refactor keg, keg_relocate and elf.rb modules to use rbelftools and patchelf.rb instead of calling shell commands. Patchelf.rb is doesn't work well with the ELFs modified using patchelf. Make patchelf.rb behave like patchelf. This is important as most of the bottles in the bintray are relocated with placeholders using patchelf.