Update .travis.yml
Update .travis.yml

trigger doc build
PistonDevelopers/image,tomaka/image,dtwood/piston-image,huonw/image,theotherphil/image,PistonDevelopers/image,JaySon-Huang/image,nwin/rust-image,hetzge/image,twoodford/image,TyOverby/rust-image,lady-segfault/image,bvssvni/image,mbrubeck/image,ruud-v-a/image