Fix gradle to not call invalid .get()

We previously failed to build ther sourcesJar when calling
`publishToMavenLocal`. This is now fixed and tested by publishing all
JARs to maven local.

build.gradle

@@ -68,7 +68,7 @@ subprojects {
     task sourcesJar(type: Jar) {
         metaInf.with(licenseSpec)
         from {
-            sourceSets.main.get().allJava
+            sourceSets.main.allJava
         }
         archiveClassifier = "sources"
     }