Fix Travis builds by adding a -p to mkdir.
Fix Travis builds by adding a -p to mkdir.

As of today, we sometimes fail Travis with "cannot create directory
/home/travis/gopath/bin: file exists". This fixes that.

sophie-h/boulder,r0ro/boulder,patf/boulder,r0ro/boulder,lucasallan/boulder,bmw/boulder,sophie-h/boulder,lucasallan/boulder,ZCloud-Firstserver/boulder,letsencrypt/boulder,benileo/boulder,r0ro/boulder,ibukanov/boulder,mozmark/boulder,mozmark/boulder,mehmooda/boulder,patf/boulder,jfrazelle/boulder,sophie-h/boulder,ibukanov/boulder,jcjones/boulder,ibukanov/boulder,mehmooda/boulder,jfrazelle/boulder,benileo/boulder,jcjones/boulder,jsha/boulder,hlandauf/boulder,jfrazelle/boulder,benileo/boulder,mozmark/boulder,ZCloud-Firstserver/boulder,hlandauf/boulder,r0ro/boulder,patf/boulder,letsencrypt/boulder,jcjones/boulder,jfrazelle/boulder,ZCloud-Firstserver/boulder,patf/boulder,ZCloud-Firstserver/boulder,hlandauf/boulder,benileo/boulder,bmw/boulder,mehmooda/boulder,mozmark/boulder,letsencrypt/boulder,lucasallan/boulder,mehmooda/boulder,bmw/boulder,jsha/boulder,jcjones/boulder,hlandauf/boulder,jsha/boulder