Extend ServletModule instead of HttpPluginModule

As described in I3b89150d1.

Change-Id: Ic91ce95719c6bccc73cc29a2139a2274f20d1939
diff --git a/src/main/java/com/googlesource/gerrit/plugins/readonly/HttpModule.java b/src/main/java/com/googlesource/gerrit/plugins/readonly/HttpModule.java
index be76f40..f27273d 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/readonly/HttpModule.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/readonly/HttpModule.java
@@ -16,10 +16,10 @@
 
 import com.google.gerrit.extensions.registration.DynamicSet;
 import com.google.gerrit.httpd.AllRequestFilter;
-import com.google.gerrit.httpd.plugins.HttpPluginModule;
 import com.google.inject.Scopes;
+import com.google.inject.servlet.ServletModule;
 
-public class HttpModule extends HttpPluginModule {
+public class HttpModule extends ServletModule {
   @Override
   protected void configureServlets() {
     DynamicSet.bind(binder(), AllRequestFilter.class).to(ReadOnly.class).in(Scopes.SINGLETON);