Add some thread safety defense

To make sure only one definition block can run at a time so that global
`describe` and `it` functions don’t get mis-attached.

src/main/java/com/greghaskins/spectrum/Spectrum.java

@@ -146,7 +146,7 @@ public void run(final RunNotifier notifier) {
         this.rootSuite.run(notifier);
     }
 
-    private static void beginDefintion(final Suite suite, final Block definitionBlock) {
+    synchronized private static void beginDefintion(final Suite suite, final Block definitionBlock) {
         suiteStack.push(suite);
         try {
             definitionBlock.run();
@@ -156,7 +156,7 @@ private static void beginDefintion(final Suite suite, final Block definitionBloc
         suiteStack.pop();
     }
 
-    private static Suite getCurrentSuite() {
+    synchronized private static Suite getCurrentSuite() {
         return suiteStack.peek();
     }