chore(travis): make docs generate on the travis leader

The leader tries to push the API docs to docs.angulardart.org, and as
such needs to have the docs built.

Author: rkirov

scripts/travis/build.sh

@@ -128,7 +128,7 @@ fi
 echo '-------------------------'
 echo '-- DOCS: Generate Docs --'
 echo '-------------------------'
-if [[ $TESTS == "dart2js" ]]; then
+if [[ $BUILD_LEADER == "YES" ]]; then
   echo $NGDART_SCRIPT_DIR/generate-documentation.sh;
   $NGDART_SCRIPT_DIR/generate-documentation.sh;
 fi