Update Visual Studio generators in documentation

README.md

@@ -594,9 +594,9 @@ process.
     where `<generator>` is `Unix Makefiles` on Unix systems and one of the
     following Visual Studio generators on Windows:
 
-    - `Visual Studio 11 Win64`
     - `Visual Studio 12 Win64`
     - `Visual Studio 14 Win64`
+    - `Visual Studio 15 Win64`
 
     Remove the `Win64` part in these generators if your Vim architecture is
     32-bit.

doc/youcompleteme.txt

@@ -807,9 +807,9 @@ will notify you to recompile it. You should then rerun the install process.
    where '<generator>' is 'Unix Makefiles' on Unix systems and one of the
    following Visual Studio generators on Windows:
 
-   - 'Visual Studio 11 Win64'
    - 'Visual Studio 12 Win64'
    - 'Visual Studio 14 Win64'
+   - 'Visual Studio 15 Win64'
 
    Remove the 'Win64' part in these generators if your Vim architecture is
    32-bit.