Add stable release branch to CI

(cherry picked from commit 0ccb690853)

.appveyor.yml

@@ -2,6 +2,7 @@ branches:
     only:
         - ci
         - master
+        - 3.3-stable
 skip_tags: true
 environment:
     CFLAGS: /WX

.travis.yml

@@ -4,6 +4,7 @@ branches:
     only:
         - ci
         - master
+        - 3.3-stable
 matrix:
     include:
         - os: linux