Fix appveyor.yml to use libyaml tag not branch

.appveyor.yml

diff --git a/.appveyor.yml b/.appveyor.yml
index a24842e..1ca6e2c 100644
--- a/.appveyor.yml
+++ b/.appveyor.yml
@@ -10,7 +10,7 @@
 
 environment:
   libyaml_repo_url: https://github.com/yaml/libyaml.git
-  libyaml_refspec: release/0.2.2
+  libyaml_refspec: 0.2.2
 #  matrix:
 #    - PYTHON_VER: Python27
 #    - PYTHON_VER: Python27-x64