Use Atmosphere JS 2.1.5.vaadin4 with Firefox fix (#13578)

Change-Id: I8d3f414a40affdcbc1d6d1210207ee9868f49f31

push/ivy.xml

@@ -3,7 +3,7 @@
 <!-- Keep the version number in sync with build.xml -->
     <!ENTITY atmosphere.runtime.version "2.1.2.vaadin2">
 
-    <!ENTITY atmosphere.js.version "2.1.5.vaadin3">
+    <!ENTITY atmosphere.js.version "2.1.5.vaadin4">
 ]>
 <ivy-module version="2.0"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"