Update Tcl/Tk to 8.6.4.

The new version of Tcl/Tk plays nice(r) with VS2015 CTP 6+.
diff --git a/PCbuild/tcltk.props b/PCbuild/tcltk.props
index f5a8643..96bd577 100644
--- a/PCbuild/tcltk.props
+++ b/PCbuild/tcltk.props
@@ -4,7 +4,7 @@
   <PropertyGroup>
     <TclMajorVersion>8</TclMajorVersion>
     <TclMinorVersion>6</TclMinorVersion>
-    <TclPatchLevel>3</TclPatchLevel>
+    <TclPatchLevel>4</TclPatchLevel>
     <TclRevision>1</TclRevision>
     <TkMajorVersion>$(TclMajorVersion)</TkMajorVersion>
     <TkMinorVersion>$(TclMinorVersion)</TkMinorVersion>