Establish a source_root for pants scala code.

This fixes source jar publication for org.pantsbuild#zinc.

Testing Done:
Noticed this while hand checking the recent zinc publication artifacts:
  http://search.maven.org/#artifactdetails%7Corg.pantsbuild%7Czinc%7C1.0.0%7Cjar

Before:
```console
$ yes | pants.dev publish.jar --local=/tmp/before --no-dryrun --force src/scala/::
$ zipinfo -1 /tmp/before/org/pantsbuild/zinc/1.0.1-SNAPSHOT/zinc-1.0.1-SNAPSHOT-sources.jar
META-INF/
META-INF/MANIFEST.MF
Main.scala
Util.scala
ZincClient.scala
FileFPrint.scala
Nailgun.scala
Compiler.scala
Options.scala
Setup.scala
Settings.scala
SbtAnalysis.scala
Cache.scala
Inputs.scala
```

After:
```console
$ yes | pants.dev publish.jar --local=/tmp/after --no-dryrun --force src/scala/::
$ zipinfo -1 /tmp/after/org/pantsbuild/zinc/1.0.1-SNAPSHOT/zinc-1.0.1-SNAPSHOT-sources.jar
META-INF/
META-INF/MANIFEST.MF
org/
org/pantsbuild/
org/pantsbuild/zinc/
org/pantsbuild/zinc/Main.scala
org/pantsbuild/zinc/Util.scala
org/pantsbuild/zinc/ZincClient.scala
org/pantsbuild/zinc/FileFPrint.scala
org/pantsbuild/zinc/Nailgun.scala
org/pantsbuild/zinc/Compiler.scala
org/pantsbuild/zinc/Options.scala
org/pantsbuild/zinc/Setup.scala
org/pantsbuild/zinc/Settings.scala
org/pantsbuild/zinc/SbtAnalysis.scala
org/pantsbuild/zinc/Cache.scala
org/pantsbuild/zinc/Inputs.scala
```

CI went green here:
  https://travis-ci.org/pantsbuild/pants/builds/61858470

Bugs closed: 1514

Reviewed at https://rbcommons.com/s/twitter/r/2189/

BUILD

@@ -5,6 +5,7 @@
 source_root('src/python', page, python_binary, python_library, resources)
 source_root('src/java', page, java_library, jvm_binary)
 source_root('src/resources', page, resources)
+source_root('src/scala', page, scala_library, jvm_binary)
 
 # Pants test code
 source_root('tests/python', page, python_library, python_tests, python_test_suite, python_binary, resources)