Prepare for release 0.13.1.

Author: yuxincs

gradle.properties

@@ -12,7 +12,7 @@ org.gradle.caching=true
 org.gradle.jvmargs=-Xmx2g -XX:MaxMetaspaceSize=512m
 
 GROUP=com.uber.nullaway
-VERSION_NAME=0.13.1-SNAPSHOT
+VERSION_NAME=0.13.1
 
 POM_DESCRIPTION=A fast annotation-based null checker for Java