Update linear-gradient mixin to spec
Update linear-gradient mixin to spec

ismarkunc/bourbon,setie/bourbon,jamesmenera/bourbon-libsass,greyhwndz/bourbon,Jazz-Man/bourbon,jasonwalkow/bourbon,jamadam/bourbon-docs-jp,zyj1022/bourbon,jamadam/bourbon-docs-jp,smithdamen/bourbon,thoughtbot/bourbon,chriseppstein/bourbon,smithdamen/bourbon,jamadam/bourbon-docs-jp,nagyistoce/sass-bourbon.io,mindeavor/bourbon,craftsmen/bourbon,thoughtbot/bourbon,chriseppstein/bourbon,craftsmen/bourbon,jamesmenera/bourbon-libsass,Nikeshsuwal/bourbon,cllns/bourbon,mindeavor/bourbon,nickg33/bourbon,nickg33/bourbon,jasonwalkow/bourbon,jamadam/bourbon-docs-jp,Jazz-Man/bourbon,zyj1022/bourbon,setie/bourbon,san650/bourbon,greyhwndz/bourbon