[ci fw-only Java/servlet3] Add missing entry for servlet3 to .travis.yml (#3539)

.travis.yml

@@ -97,6 +97,7 @@ env:
      - "TESTDIR=Java/restexpress"
      - "TESTDIR=Java/revenj-jvm"
      - "TESTDIR=Java/servlet"
+     - "TESTDIR=Java/servlet3"
      - "TESTDIR=Java/spark"
      - "TESTDIR=Java/spring"
      - "TESTDIR=Java/tapestry"