Gradle release v1.8.0 (#2154)

* [Gradle Release Plugin] - pre tag commit:  'v1.8.0-gradle'.

* [Gradle Release Plugin] - new version commit:  'v1.8.1-SNAPSHOT-gradle'.

Author: chanseokoh

jib-gradle-plugin/gradle.properties

@@ -1 +1 @@
-version = 1.7.1-SNAPSHOT
+version = 1.8.1-SNAPSHOT