Switch MSVC from /Wall to /W4

Apparently 4 is recommended by Microsoft!

https://msdn.microsoft.com/en-us/library/thxezb7y.aspx

Author: alexcrichton

src/lib.rs

@@ -199,7 +199,7 @@ impl ToolFamily {
 
     /// What the flags to enable all warnings
     fn warnings_flags(&self) -> &'static [&'static str] {
-        static MSVC_FLAGS: &'static [&'static str] = &["/Wall"];
+        static MSVC_FLAGS: &'static [&'static str] = &["/W4"];
         static GNU_CLANG_FLAGS: &'static [&'static str] = &["-Wall", "-Wextra"];
 
         match *self {