defineCompiler should be optional

[slandelle]

I guess some people might have to configure a specific compiler version, but by default, the plugin should be able to infer the proper scala-compiler artifact (groupId and artifactId are well known and version can be inferred from the scala-library jar that's in the project dependency tree).

(that's what I do in the gatling-maven-plugin)

[Andrei-Pozolotin]

see #7

[slandelle]

Looking good as of 1.2.4.20180217070934