[travis] track an 8.7 specific branch of HoTT.
[travis] track an 8.7 specific branch of HoTT.

This is required since we merged PR#309: Ltac as a plugin.

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