Run tests against examples & compile starter code

- adjusted "etl" as it has "starter" code.

.travis.yml

 
 script:
   - gradle --version
-  - TERM=dumb gradle check --continue
+  - TERM=dumb gradle check compileStarterSourceJava --continue
   - bin/fetch-configlet
   - bin/configlet .

build.gradle

-subprojects {
+subprojects { project ->
   apply plugin: "java"
 
   sourceSets {
+    // Verify that the tests are working by running them against the example code.
+    // By replacing the "main" sourceSet with the example code we avoid any collisions
+    //   with solution code that may have been included as a jumpstart (e.g. etl).
     main {
-      java.srcDirs = ['src/example/java']
+      java.srcDirs = ["src/example/java"]
+    }
+    println "Source directory for " + project.name + "'s main source set: " + main.java.srcDirs
+
+    starterSource {
+      java.srcDirs = ["src/main/java"]
     }
   }
 }

settings.gradle

-include 'robot-name', 'prime-factors'
+include 'etl', 'robot-name', 'prime-factors'