Build /permissive- under VS2017

Building with the (VS2017) /permissive- flag puts the compiler into
stricter standards-compliant mode.  It shouldn't cause the compiler to
work differently--it just disallows some non-conforming code--so should
be perfectly fine for the test suite under all VS2017 builds.

This commit also fixes one failure under non-permissive mode.

.appveyor.yml

@@ -34,6 +34,7 @@ install:
     if ($env:APPVEYOR_JOB_NAME -like "*Visual Studio 2017*") {
       $env:CMAKE_GENERATOR = "Visual Studio 15 2017"
       $env:CMAKE_INCLUDE_PATH = "C:\Libraries\boost_1_64_0"
+      $env:CXXFLAGS = "-permissive-"
     } else {
       $env:CMAKE_GENERATOR = "Visual Studio 14 2015"
     }

include/pybind11/cast.h

@@ -1417,7 +1417,7 @@ protected:
     bool load_value(value_and_holder &&v_h) {
         if (v_h.holder_constructed()) {
             value = v_h.value_ptr();
-            holder = v_h.holder<holder_type>();
+            holder = v_h.template holder<holder_type>();
             return true;
         } else {
             throw cast_error("Unable to cast from non-held to held instance (T& to Holder<T>) "