Upgrade user agent version to 1.6

Author: kevinsawicki

app/src/main/java/com/github/mobile/DefaultClient.java

@@ -29,7 +29,7 @@
  */
 public class DefaultClient extends GitHubClient {
 
-    private static final String USER_AGENT = "GitHubAndroid/1.4";
+    private static final String USER_AGENT = "GitHubAndroid/1.6";
 
     static {
         // Disable http.keepAlive on Froyo and below