Fix error introduced by when resolving conflict

It's possible for `config.excludeNodeModules` to be undefined, this is viewed by testExclude as 'not true'.

index.js

@@ -58,7 +58,7 @@ function NYC (config) {
     cwd: this.cwd,
     include: config.include,
     exclude: config.exclude,
-    excludeNodeModules: config.excludeNodeModules,
+    excludeNodeModules: config.excludeNodeModules !== false,
     extension: this.extensions
   })