Assign Gravatar as default avatar generator

The latest Gitblit 1.7 (develop) requires to have at least one
avatar generator configured.
The only one provided at the moment with GitBlit is Gravatar so we
need to make sure that it is bound during the Guice module set-up
in order to avoid runtime errors.

Change-Id: I886352258029e6dc801bbbca3376e0f8ffd38e81
diff --git a/src/main/java/com/googlesource/gerrit/plugins/gitblit/GitBlitServletModule.java b/src/main/java/com/googlesource/gerrit/plugins/gitblit/GitBlitServletModule.java
index 9012053..11946bf 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/gitblit/GitBlitServletModule.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/gitblit/GitBlitServletModule.java
@@ -18,6 +18,8 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import com.gitblit.AvatarGenerator;
+import com.gitblit.GravatarGenerator;
 import com.gitblit.IStoredSettings;
 import com.gitblit.guice.IPublicKeyManagerProvider;
 import com.gitblit.guice.ITicketServiceProvider;
@@ -73,6 +75,7 @@
     log.info("Configuring Gitblit core services");
     bind(IStoredSettings.class).to(GitBlitSettings.class);
     bind(XssFilter.class).to(JSoupXssFilter.class);
+    bind(AvatarGenerator.class).to(GravatarGenerator.class);
 
     // bind complex providers
     bind(IPublicKeyManager.class).toProvider(IPublicKeyManagerProvider.class);