Update to travel sponsorship policy

The Travel Committee have traditionally had a policy that local travel
was not reimbursed unless explicitly agreed beforehand. The policy was
not explicitly mentioned anywhere, so I have updated the wiki to make
it more clear: https://wiki.gnome.org/Travel#Travel.2BAC8-Policy.Travel_policy

This applies to large events which are handled by the Travel Committee
and to small events which are handled by the Foundation board.