Use external properties if env variables are not defined.

Author: apottere

build.gradle

@@ -156,8 +156,8 @@ release {
 afterReleaseBuild.dependsOn bintrayUpload
 
 bintray {
-    user = System.getenv('BINTRAY_USER')
-    key = System.getenv('BINTRAY_KEY')
+    user = System.getenv('BINTRAY_USER') ?: BINTRAY_USER
+    key = System.getenv('BINTRAY_KEY') ?: BINTRAY_KEY
     publications = ['maven']
     publish = true
     pkg {