INSTALL.Windows.md: Update the minimum required compiler.
MSVC 2013 update 4 was released in Nov 2014. Its release notes
says that it has fixed the problem wherein using designated
initializers to initialize unions within structs would
fail to compile.
Using designated initializers is useful in OVS and so this commit
updates the minimum required compiler for Windows.
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Acked-by: Ben Pfaff <blp@nicira.com>