fix(vue2-jest): interop custom transformer require

interop custom transformer require. Some transformers may not
interop correctly when requiring, return `transformer.default` or `transformer`.

fixes vuejs#383

packages/vue2-jest/lib/utils.js

@@ -104,6 +104,7 @@ const getCustomTransformer = function getCustomTransformer(
     require(resolvePath(transformerPath))
   ) {
     transformer = require(resolvePath(transformerPath))
+    transformer = transformer.default || transformer
   } else if (typeof transformerPath === 'object') {
     transformer = transformerPath
   }