Mention that VS2013 is needed

Fixes rust-lang/rust#29479

Author: steveklabnik

downloads.html

@@ -184,7 +184,7 @@ <h2>Nightly&nbsp; (1.6)</h2>
 	</p>
 	<p>
 	  MSVC builds of Rust additionally require an
-	  <a href="https://www.visualstudio.com/downloads">installation of Visual Studio</a>
+	  <a href="https://www.visualstudio.com/downloads">installation of Visual Studio 2013</a>
 	  so rustc can use its linker.
 	  No additional software installation is necessary for basic use of the GNU build.
 	</p>