Make SBT build relative to SBT's baseDirectory

[ComFreek]

to allow other SBT projects to reference MMT as a subproject.

Devs of MMT can now just do:

1. Clone MMT/devel somewhere
2. Set up an empty SBT project somewhere else (!) with the following build.sbt:

import sbt.Keys.libraryDependencies

lazy val mmt = RootProject(file("<path-to-mmt-clone>/src"))

lazy val playground = Project(id = "playground", base = file(".")).settings(
    name := "playground",
    version := "0.1",
    scalaVersion := "2.12.8",
    scalacOptions in ThisBuild ++= Seq("-unchecked", "-deprecation"),
    // Add further desired libraryDependencies here
        // e.g. libraryDependencies += "org.jgrapht" % "jgrapht-core" % "1.3.0",
).dependsOn(mmt).aggregate(mmt)

// Deploy MMT JAR via simply "deploy" in the playground SBT shell.

This commit tremendously alleviates #442.

[tkw1536]

Note: The Travis CI build only fails the content tests, the actual building seems to work. I see nothing against merging this.