oasis: Change instances of 'False' to 'false'

This keeps happening. Luckily, False will be recognized as a false value
in the next release of oasis. See:

  ocaml/oasis#33
oasis: check flag(async) for example programs

These programs should not build unless the async flag is true, as they
use the openflow.async subpackage.
Merge pull request #76 from frenetic-lang/oasis

Oasis fixes