Drop dependency on camlp4 and use ppx

• Ported to ppx instead of camlp4. This bumps up the minimum version
  of OCaml required to 4.02.3, but removes upper bounds on libraries (@avsm)
• Improved Travis CI test coverage to use Docker for multi-distro
  Linux support (@avsm)
• Add Alpine Linux depexts for linux-kernel-headers (@avsm)