[default.nix] Use OCaml 4.06
[default.nix] Use OCaml 4.06

ppedrot/coq,maximedenes/coq,ejgallego/coq,Zimmi48/coq,Matafou/coq,silene/coq,coq/coq,silene/coq,Zimmi48/coq,ppedrot/coq,maximedenes/coq,Matafou/coq,gares/coq,gares/coq,coq/coq,gares/coq,maximedenes/coq,coq/coq,Zimmi48/coq,SkySkimmer/coq,maximedenes/coq,ejgallego/coq,ejgallego/coq,gares/coq,Zimmi48/coq,SkySkimmer/coq,Matafou/coq,SkySkimmer/coq,gares/coq,coq/coq,JasonGross/coq,SkySkimmer/coq,ppedrot/coq,ejgallego/coq,maximedenes/coq,JasonGross/coq,ppedrot/coq,silene/coq,JasonGross/coq,JasonGross/coq,ppedrot/coq