Declare that 2.1 is the last version to support Python 2.6 (#3944)

* Declare that 2.1 is the last version to support Python 2.6

* It's the Final Countdown!
diff --git a/docs/installation.rst b/docs/installation.rst
index 51caefd..7066865 100644
--- a/docs/installation.rst
+++ b/docs/installation.rst
@@ -22,8 +22,8 @@
 * 32-bit and 64-bit Python on 64-bit Windows Server 2012
 
 .. warning::
-    Python 2.6 is no longer supported by the Python core team. A future version
-    of cryptography will drop support for this version.
+    Python 2.6 is no longer supported by the Python core team. The next release
+    of ``cryptography`` will drop support for Python 2.6.
 
 We test compiling with ``clang`` as well as ``gcc`` and use the following
 OpenSSL releases: