Fix OS X Travis by pinning OCaml version.
Fix OS X Travis by pinning OCaml version.

JasonGross/coq,Matafou/coq,william-lawvere/coq,letouzey/coq-wip,Zimmi48/coq,Matafou/coq,coq/coq,ppedrot/coq,letouzey/coq-wip,letouzey/coq-wip,coq/coq,amintimany/coq,amintimany/coq,ppedrot/coq,ppedrot/coq,coq/coq,Matafou/coq,william-lawvere/coq,Zimmi48/coq,ejgallego/coq,maximedenes/coq,Zimmi48/coq,JasonGross/coq,silene/coq,SkySkimmer/coq,letouzey/coq-wip,ejgallego/coq,gares/coq,Zimmi48/coq,maximedenes/coq,ejgallego/coq,letouzey/coq-wip,SkySkimmer/coq,silene/coq,ejgallego/coq,gares/coq,william-lawvere/coq,SkySkimmer/coq,gares/coq,JasonGross/coq,maximedenes/coq,JasonGross/coq,coq/coq,silene/coq,gares/coq,maximedenes/coq,ppedrot/coq,ppedrot/coq,SkySkimmer/coq,maximedenes/coq,amintimany/coq,gares/coq