[travis] Add webhook to Gitter.
[travis] Add webhook to Gitter.

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