Make sure that reference doc is served static

Newer versions of the App Engine SDK insist that we declare our
statically served files. We omitted /reference/**. This commit adds it.

Change-Id: I652adae99b7bf3df50ea8b6fb3f201c430d3e002

appinventor/appengine/war/WEB-INF/appengine-web.xml

@@ -18,6 +18,7 @@
     <include path="/gwt.css" expiration="1d" />
     <include path="/Ya.css" expiration="1d" />
     <include path="/templates/**" expiration="1d" />
+    <include path="/reference/**" expiration="1d" />
     <include path="/closure-library/closure/goog/base.js" expiration="1d" />
     <include path="/closure-library/closure/goog/css/dialog.css" expiration="1d" />
     <include path="/closure-library/closure/goog/deps.js" expiration="1d" />