Bump Guava to 21.0

Change-Id: Id81705773d191db6a18153d8b2dd4abd2bfd0e60
diff --git a/lib/pom.xml b/lib/pom.xml
index c7616bc..1462c01 100644
--- a/lib/pom.xml
+++ b/lib/pom.xml
@@ -46,7 +46,7 @@
         <jetty.version>9.2.21.v20170120</jetty.version>
         <jackson.version>2.8.6</jackson.version>
         <slf4j.version>1.7.21</slf4j.version>
-        <guava.version>20.0</guava.version>
+        <guava.version>21.0</guava.version>
         <commons.io.version>2.4</commons.io.version>
         <!-- TODO argLine was originally added maven-surfire-plugin configuration
                   to fix locale errors for non-US developers. However, it breaks