Update gradle.lockfile

Author: hiroyuki-sato

gradle.lockfile

@@ -12,7 +12,7 @@ org.embulk:embulk-util-config:0.5.0=compileClasspath,runtimeClasspath
 org.embulk:embulk-util-file:0.2.0=compileClasspath,runtimeClasspath
 org.embulk:embulk-util-json:0.5.0=compileClasspath,runtimeClasspath
 org.embulk:embulk-util-rubytime:0.4.0=compileClasspath,runtimeClasspath
-org.embulk:embulk-util-timestamp:0.3.0=compileClasspath,runtimeClasspath
+org.embulk:embulk-util-timestamp:0.3.1=compileClasspath,runtimeClasspath
 org.msgpack:msgpack-core:0.8.24=compileClasspath
 org.slf4j:slf4j-api:2.0.7=compileClasspath
 empty=