Update to GWT version 2.7.0.vaadin3.

The fix in the new GWT version is
https://dev.vaadin.com/review/gitweb?p=gwt.git;a=commit;h=d6116973b0b211e50243efe5f88aa893ffdbd037
(GWT RPC fix addressing a memory leak on IE9)

Change-Id: I39f3a17167cfd69c2b90d0f225a967a281fd9ef5

build.properties

@@ -6,5 +6,5 @@ vaadin.url=http://vaadin.com
 vaadin.java.version=1.6
 vaadin.version=0.0.0.unversioned-development-build
 vaadin.sass.version=0.9.12
-gwt.version=2.7.0.vaadin2
+gwt.version=2.7.0.vaadin3
 commons-io.version=2.4