Port to dune

• opam: updated bounds on a more conservative basis
• travis: tests more compilers
• tests: disable useless-object-inheritance on pylint checks
• pythongen: generate python2-3 compatible bindings
• Add ISC license
• Incremented the upper bound for async's version.
• Added lower bound for js_of_ocaml in related .opam file.
• Fixed compilation issue with js_of_ocaml 3.5.0 and 3.5.1.
• opam: remove the 'build' directive on dune dependency
• opam: remove unnecessary flag
• port to dune