Upgrade guava and gson to their latest versions

Change-Id: I60c5315078db02af64c9063c91bfd8905d53f375
diff --git a/app/build.gradle b/app/build.gradle
index 338f95c..6430dcb 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -26,8 +26,8 @@
 
     compile 'com.github.uwolfer:gerrit-rest-java-client:dd4837b'
 
-    compile 'com.google.guava:guava:19.0'
-    compile 'com.google.code.gson:gson:2.6.2'
+    compile 'com.google.guava:guava:20.0'
+    compile 'com.google.code.gson:gson:2.8.0'
 
     final ANDROID_SUPPORT_VERSION = '25.0.1'
     compile "com.android.support:appcompat-v7:${ANDROID_SUPPORT_VERSION}"