Not compatible with Gradle 5.1

The plugin fails when it's invoked by Gradle 5.1:

```
java.lang.NoSuchMethodError: org.gradle.api.tasks.compile.CompileOptions.getBootClasspath()Ljava/lang/String;
        at me.tatarka.RetrolambdaTransform.getClasspath(RetrolambdaTransform.groovy:126)
```