travis: prefer-dist as newly recommended by composer
travis: prefer-dist as newly recommended by composer

https://twitter.com/seldaek/status/708236348281495552
fruux/sabre-dav,PVince81/sabre-dav,kriswebdev/sabre-dav,staabm/sabre-dav,staabm/sabre-dav,fruux/sabre-dav,c0d3z3r0/sabre-dav,PVince81/sabre-dav,c0d3z3r0/sabre-dav,staabm/sabre-dav,kriswebdev/sabre-dav,kriswebdev/sabre-dav,fruux/sabre-dav,c0d3z3r0/sabre-dav,PVince81/sabre-dav