Travis: Run the javadoc command only the on the main compiler project…

…, not the GWT build.

We only run this to make sure a javadoc failure won't cause errors when trying to do a release. This isn't needed for the GWT build since it isn't released through Maven.

Fixes #2568

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=161694147

.travis.yml

@@ -15,7 +15,7 @@ script:
   # Builds the compiler and runs tests
   - mvn install --batch-mode
   # Checks for JavaDoc errors
-  - mvn javadoc:javadoc
+  - mvn -pl com.google.javascript:closure-compiler javadoc:javadoc
 
 # Causes the build to run on a VM instead of in a container, to avoid OOM errors.
 sudo: required