Update ErrorProne to 2.1.3 and fix failures

The fixes could have subtle side-effects, but I did take care when making them.
diff --git a/build.gradle b/build.gradle
index 8d9768e..80e0deb 100644
--- a/build.gradle
+++ b/build.gradle
@@ -35,7 +35,7 @@
       dependencies {
         // The ErrorProne plugin defaults to the latest, which would break our
         // build if error prone releases a new version with a new check
-        errorprone 'com.google.errorprone:error_prone_core:2.0.21'
+        errorprone 'com.google.errorprone:error_prone_core:2.1.3'
         apt 'com.google.guava:guava-beta-checker:1.0'
       }
     } else {
@@ -76,7 +76,8 @@
 
     compileTestJava {
       // serialVersionUID is basically guaranteed to be useless in our tests
-      options.compilerArgs += ["-Xlint:-serial"]
+      // LinkedList doesn't hurt much in tests and has lots of usages
+      options.compilerArgs += ["-Xlint:-serial", "-Xep:JdkObsolete:OFF"]
     }
 
     jar.manifest {