Upgrade to jruby/gradle core 1.0.4 with some more recent bugfixes

This commit also changes JRubyStormJar a bit to override properties now that
they are properly overwritable in 1.0.4

Fixes #22

gradle.properties

@@ -4,4 +4,4 @@ bintrayKey=
 sourceCompatibility=1.7
 targetCompatibility=1.7
 
-jrubyGradleMinVersion=1.0.3
+jrubyGradleMinVersion=1.0.4

src/main/groovy/com/github/jrubygradle/storm/internal/JRubyStormJar.groovy

@@ -31,11 +31,17 @@ class JRubyStormJar extends JRubyJar {
         return '1.7.22'
     }
 
-    String mainClass = REDSTORM_MAIN
+    @Override
+    String getMainClass() {
+        return REDSTORM_MAIN
+    }
 
     @Override
     String getConfiguration() {
-        return parentTask.configuration.name
+        if (parentTask) {
+            return parentTask.configuration.name
+        }
+        return JRubyStorm.DEFAULT_CONFIGURATION_NAME
     }
 
     JRubyStormJar() {