avoid deprecated method since sbt 0.13.13
avoid deprecated method since sbt 0.13.13

```
build.sbt:168: warning: `<<=` operator is deprecated. Use `key := { x.value }` or `key ~= (old => { newValue })`.
See http://www.scala-sbt.org/0.13/docs/Migrating-from-sbt-012x.html
publishTo <<= version { (v: String) =>
          ^
```

takezoe/gitbucket,x-way/gitbucket,superhj1987/gitbucket,s-takatsu/gitbucket,xuwei-k/gitbucket,zhoffice/gitbucket,takezoe/gitbucket,rabitarochan/gitbucket,superhj1987/gitbucket,nobusugi246/gitbucket,kounoike/gitbucket,rabitarochan/gitbucket,shiena/gitbucket,McFoggy/gitbucket,imeszaros/gitbucket,gencer/gitbucket,shiena/gitbucket,s-takatsu/gitbucket,shiena/gitbucket,zhoffice/gitbucket,s-takatsu/gitbucket,nus/gitbucket,nobusugi246/gitbucket,McFoggy/gitbucket,mann-ed/gitbucket,gencer/gitbucket,x-way/gitbucket,imeszaros/gitbucket,nobusugi246/gitbucket,gitbucket/gitbucket,rabitarochan/gitbucket,gitbucket/gitbucket,superhj1987/gitbucket,kounoike/gitbucket,xuwei-k/gitbucket,zhoffice/gitbucket,takezoe/gitbucket,kounoike/gitbucket,gencer/gitbucket,x-way/gitbucket,superhj1987/gitbucket,mann-ed/gitbucket,nus/gitbucket,imeszaros/gitbucket,McFoggy/gitbucket,shiena/gitbucket,gencer/gitbucket,mann-ed/gitbucket,zhoffice/gitbucket,kounoike/gitbucket,rabitarochan/gitbucket,x-way/gitbucket,gitbucket/gitbucket,nus/gitbucket,takezoe/gitbucket,gitbucket/gitbucket,nus/gitbucket,xuwei-k/gitbucket,s-takatsu/gitbucket,imeszaros/gitbucket,nobusugi246/gitbucket,xuwei-k/gitbucket,McFoggy/gitbucket,mann-ed/gitbucket