Removed an unneeded typename in dynamic.h

Summary: This `typename` qualifier was unneeded, and MSVC errors if it's there, so remove it.
Closes #275

Reviewed By: @yfeldblum

Differential Revision: D2284056

Pulled By: @sgolemon

folly/dynamic.h

@@ -532,7 +532,7 @@ private:
      * incomplete type right now).  (Note that in contrast we know it
      * is ok to do this with fbvector because we own it.)
      */
-    typename std::aligned_storage<
+    std::aligned_storage<
       sizeof(std::unordered_map<int,int>),
       alignof(std::unordered_map<int,int>)
     >::type objectBuffer;