update publishing scheme to be able to publish with signed artifacts

publish.gradle

@@ -14,6 +14,10 @@ bintray {
             name = "${project.version}"
             released = new Date()
             vcsTag = "${project.version}"
+            gpg {
+                sign = true
+                passphrase = project.hasProperty('signing.gnupg.passphrase') ? project.property('signing.gnupg.passphrase') : System.getenv('signing.gnupg.passphrase')
+            }
         }
     }
 }