Remove restriction on emoji with - in their code.
Remove restriction on emoji with - in their code.

This makes pretty much all slack emoji send *something* to matrix. Sometimes
however it is not the correct emoji. I can't decern the pattern of what works
and what doesn't though.

matrix-org/matrix-appservice-slack,matrix-org/matrix-appservice-slack,matrix-org/matrix-appservice-slack,matrix-org/matrix-appservice-slack