Use opam pin for development
opam: make quickcheck an optional dependency
opam: only specify major opam version

Doing otherwise trips up earlier versions of opam, even though the file
is compatible with them.
opam: update README
opam: update travis build matrix
opam: skip FRENETIC_DEPENDS

There are none.
Merge pull request #34 from frenetic-lang/opam

Use opam pin for development