[release] Release geldata 0.4.0. (#59)

Author: dnwpark

build.gradle

@@ -5,7 +5,7 @@ plugins {
 }
 
 ext {
-    project_version = '0.4.0-SNAPSHOT'
+    project_version = '0.4.0'
     github_org = 'geldata'
     project_name = 'gel-java'
     artifact_group = 'com.geldata'