Scalafix's scalahost dependency should depend on scala version

[lolgab]

cbt/plugins/scalafix/build/build.scala

Line 21 in 663a0f9

"org.scalameta", "scalahost_" ++ scalaVersion, "1.6.0"

Now the scalahost dependency is fixed but if you set the defaultScalaVersion = "2.12.3" for example the only version on maven is the "1.8.0" Maven central ..
For scala 2.12.2
I don't know what is the best way to achieve that... Maybe a Map in the plugin/scalafix/build/build.scala file ... If it is a accepted solution I can submit a PR.

Lorenzo :)