Link to changelog from runtime.txt deprecation message (#1747)

Now that the changelog post exists:
https://devcenter.heroku.com/changelog-items/3141

Author: edmorley

bin/compile

@@ -149,7 +149,8 @@ if [[ "${python_version_origin}" == "runtime.txt" ]]; then
 		Warning: The runtime.txt file is deprecated.
 
 		The runtime.txt file is deprecated since it has been replaced
-		by the more widely supported .python-version file.
+		by the more widely supported .python-version file:
+		https://devcenter.heroku.com/changelog-items/3141
 
 		Please delete your runtime.txt file and create a new file named:
 		.python-version

spec/hatchet/python_update_warning_spec.rb

@@ -18,7 +18,8 @@
           remote:  !     Warning: The runtime.txt file is deprecated.
           remote:  !     
           remote:  !     The runtime.txt file is deprecated since it has been replaced
-          remote:  !     by the more widely supported .python-version file.
+          remote:  !     by the more widely supported .python-version file:
+          remote:  !     https://devcenter.heroku.com/changelog-items/3141
           remote:  !     
           remote:  !     Please delete your runtime.txt file and create a new file named:
           remote:  !     .python-version

spec/hatchet/python_version_spec.rb

@@ -389,7 +389,8 @@
           remote:  !     Warning: The runtime.txt file is deprecated.
           remote:  !     
           remote:  !     The runtime.txt file is deprecated since it has been replaced
-          remote:  !     by the more widely supported .python-version file.
+          remote:  !     by the more widely supported .python-version file:
+          remote:  !     https://devcenter.heroku.com/changelog-items/3141
           remote:  !     
           remote:  !     Please delete your runtime.txt file and create a new file named:
           remote:  !     .python-version