all: update to guava 20

diff --git a/build.gradle b/build.gradle
index cd848b0..495d353 100644
--- a/build.gradle
+++ b/build.gradle
@@ -61,7 +61,7 @@
         protocPluginBaseName = 'protoc-gen-grpc-java'
         javaPluginPath = "$rootDir/compiler/build/exe/java_plugin/$protocPluginBaseName$exeSuffix"
 
-        guavaVersion = '19.0'
+        guavaVersion = '20.0'
         protobufVersion = '3.1.0'
         protobufNanoVersion = '3.0.0-alpha-5'