Update gradle config to support local Maven repository

When building the plugin with a locally built SNAPSHOT version of
the plugin API, it is neccessary to consume the API artifact from
the local Maven repository.

Add the necessary setting in the gradle configuration to make this
possible.

Change-Id: Ib6618572e69e1139baf8ba85e921e100045dca49
diff --git a/build.gradle b/build.gradle
index 5429d57..6792639 100644
--- a/build.gradle
+++ b/build.gradle
@@ -48,6 +48,7 @@
 }
 
 repositories {
+  mavenLocal()
   mavenCentral()
   maven {
     name = "Gerrit API repository"