Update guava version. (#118)

Author: kurtisvg

core/pom.xml

@@ -36,7 +36,7 @@
         <dependency>
             <groupId>com.google.guava</groupId>
             <artifactId>guava</artifactId>
-            <version>20.0</version>
+            <version>26.0-android</version>
         </dependency>
 
         <dependency>