DRY up SBT version Travis uses with $SBT_VERSION
DRY up SBT version Travis uses with $SBT_VERSION

rossabaker/http4s,aeons/http4s,reactormonk/http4s,aeons/http4s,rossabaker/http4s,ChristopherDavenport/http4s,http4s/http4s,ZizhengTai/http4s,ZizhengTai/http4s,reactormonk/http4s,ChristopherDavenport/http4s,ChristopherDavenport/http4s,aeons/http4s,ZizhengTai/http4s,reactormonk/http4s