travis: extend opam timeout
travis: extend opam timeout

Let opam continue when running >10minutes without output, like it happens when a single package install runs that long.

This extends timeout to 20 minutes. `travis_wait $N` would give N minutes.
https://docs.travis-ci.com/user/common-build-problems/#build-times-out-because-no-output-was-received

This would help for instance with https://travis-ci.org/ocaml/opam-repository/jobs/596032476.
ocaml/opam-repository,codinuum/opam-repository,Chris00/opam-repository,kkirstein/opam-repository,jonludlam/opam-repository,seliopou/opam-repository,Chris00/opam-repository,mjambon/opam-repository,jhwoodyatt/opam-repository,toots/opam-repository,talex5/opam-repository,talex5/opam-repository,fpottier/opam-repository,camlspotter/opam-repository,avsm/opam-repository,pirbo/opam-repository,yallop/opam-repository,djs55/opam-repository,dbuenzli/opam-repository,astrada/opam-repository,avsm/opam-repository,emillon/opam-repository,astrada/opam-repository,lefessan/opam-repository,jhwoodyatt/opam-repository,Leonidas-from-XIV/opam-repository,hannesm/opam-repository,pirbo/opam-repository,mjambon/opam-repository,dra27/opam-repository,dbuenzli/opam-repository,orbitz/opam-repository,abate/opam-repository,fpottier/opam-repository,emillon/opam-repository,ocaml/opam-repository,djs55/opam-repository,toots/opam-repository,dra27/opam-repository,yallop/opam-repository,Leonidas-from-XIV/opam-repository,nberth/opam-repository,AltGr/opam-repository,AltGr/opam-repository,pqwy/opam-repository,andersfugmann/opam-repository,c-cube/opam-repository,andersfugmann/opam-repository,kkirstein/opam-repository,camlspotter/opam-repository,pqwy/opam-repository,arlencox/opam-repository,hannesm/opam-repository,seliopou/opam-repository,nberth/opam-repository,c-cube/opam-repository,lefessan/opam-repository,hakuch/opam-repository,cakeplus/opam-repository,jonludlam/opam-repository,codinuum/opam-repository