[common/vector] reduce applicability for clang workaround

dune/stuff/common/vector.hh

@@ -255,9 +255,9 @@ operator<<(std::basic_ostream<CharType, CharTraits>& out, const V& vec)
 
 namespace std {
 /// clang 3.6 does not consider the overload in the ns for some reason during resultion of a call in gtest
-template <class V, class CharType, class CharTraits>
-typename std::enable_if<Dune::Stuff::Common::is_vector<V>::value, std::basic_ostream<CharType, CharTraits>&>::type
-operator<<(std::basic_ostream<CharType, CharTraits>& out, const V& vec)
+template <class V, class Alloc, class CharType, class CharTraits>
+std::basic_ostream<CharType, CharTraits>& operator<<(std::basic_ostream<CharType, CharTraits>& out,
+                                                     const std::vector<V, Alloc>& vec)
 {
   ::operator<<(out, vec);
   return out;