Add error_prone_annotations

This allows jclouds to compile with Guava 20.  Earlier versions
implicitly provided this.  Reference:

https://github.com/google/guava/issues/2837

project/pom.xml

@@ -362,6 +362,12 @@
     </dependencies>
   </dependencyManagement>
   <dependencies>
+    <dependency>
+      <groupId>com.google.errorprone</groupId>
+      <artifactId>error_prone_annotations</artifactId>
+      <version>2.1.0</version>
+      <scope>provided</scope>
+    </dependency>
     <dependency>
       <groupId>org.testng</groupId>
       <artifactId>testng</artifactId>