move Google Guava to mage-root pom in dependencyManagement, so other modules can use it

pom.xml

@@ -109,6 +109,11 @@
                 <artifactId>slf4j-log4j12</artifactId>
                 <version>1.8.0-beta2</version>
             </dependency>
+            <dependency>
+                <groupId>com.google.guava</groupId>
+                <artifactId>guava</artifactId>
+                <version>20.0</version>
+            </dependency>
         </dependencies>
     </dependencyManagement>
 </project>