inline flags caused build issues

[alexjbest]

I'm a complete scala novice so take this with a pinch of salt, but I wanted to post this incase it helps anyone else who couldn't build trepplein.

Running sbt stage gave me numerous errors which lead me to https://github.com/scala/bug/issues/11247 following the suggestion there and removing inline flags then re-running the build worked.

diff --git a/build.sbt b/build.sbt
index d537e73..5d49ac3 100644
--- a/build.sbt
+++ b/build.sbt
@@ -14,7 +14,7 @@ libraryDependencies += "org.specs2" %% "specs2-core" % "4.2.0" % "test"
 
 scalacOptions ++= {
   if (!scalaVersion.value.startsWith("2.12.")) Seq()
-  else Seq("-opt:l:inline", "-opt-inline-from:**", "-opt-warnings")
+  else Seq("-opt-warnings")
 }