Ensure that setting of the jrubyVersion for the topology jar works properly

This contains some workarounds until jruby-gradle/redstorm#11 is fixed which
will allow us to work on JRuby 9k, until then we'll default to the latest
stable JRuby 1.7.x branch

Fixes #23

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

@@ -16,9 +16,22 @@ class JRubyStormJar extends JRubyJar {
     /** parent from which this task will inherit some configuration */
     JRubyStorm parentTask
 
+    /**
+     * Return the version of JRuby safe for usage in redstorm
+     */
+    @Override
+    String getJrubyVersion() {
+        final String inheritedVersion = super.getJrubyVersion()
+
+        /* if our parent has a default version that's 1.7.x, use it */
+        if (inheritedVersion.matches(/1.7.(\d+)/)) {
+            return inheritedVersion
+        }
+        /*  Default to 1.7.22 <https://github.com/jruby-gradle/redstorm/issues/11> */
+        return '1.7.22'
+    }
+
     String mainClass = REDSTORM_MAIN
-    /* Default to 1.7.22 <https://github.com/jruby-gradle/redstorm/issues/11> */
-    String jrubyVersion = '1.7.22'
 
     @Override
     String getConfiguration() {

src/test/groovy/com/github/jrubygradle/storm/internal/JRubyStormJarSpec.groovy

@@ -37,4 +37,25 @@ class JRubyStormJarSpec extends Specification {
         expect: "that it has no special appendix in the filename"
         task.appendix == ''
     }
+
+    @Issue('https://github.com/jruby-gradle/redstorm/issues/11')
+    @Issue('https://github.com/jruby-gradle/jruby-gradle-storm-plugin/issues/23')
+    def 'jrubyVersion should be default to 1.7.xx'() {
+        given:
+        JRubyStormJar task = project.task('spock', type: JRubyStormJar)
+
+        expect:
+        task.jrubyVersion == '1.7.22'
+    }
+
+    def "Setting jrubyVersion should override the default"() {
+        given:
+        final String version = '1.7.21'
+        JRubyStormJar task = project.task('spock', type: JRubyStormJar) {
+            jrubyVersion version
+        }
+
+        expect:
+        task.jrubyVersion == version
+    }
 }