Build: Pin googletest to a version that works with visual studio 2013.

.appveyor.yml

@@ -31,8 +31,11 @@ matrix:
 
 # scripts that run after cloning repository
 install:
-  - git clone https://github.com/google/googletest.git External/googletest
   - C:/Python27/python.exe update_glslang_sources.py
+  - git clone https://github.com/google/googletest.git External/googletest
+  - cd External/googletest
+  - git checkout 440527a61e1c91188195f7de212c63c77e8f0a45
+  - cd ../..
 
 build:
   parallel: true  # enable MSBuild parallel builds