Don’t require that a .merlin exists

[lthms]

Merlin now uses 'dune' instead of .merlin files, and as a consequence,
this check disables 'flycheck-ocaml' on recent projects.

[bbatsov]

Ops, it seems I forgot to remove this here. Still, shouldn't we do at least a check for a dune-project instead? It seems Merlin is useless without one.

[lthms]

Makes sense. Pushed an amended commit!

[bbatsov]

Thanks and sorry for my oversight that caused this problem!

[lthms]

No problem. Thanks for maintaining flycheck-ocaml :)