chore: Pin appveyor bundler version to 1.17.3 (#66)

Author: sthulb

.appveyor.yml

@@ -20,7 +20,8 @@ install:
 - "%PYTHON%\\python.exe -m pip install -r requirements/dev.txt"
 - "%PYTHON%\\python.exe -m pip install -e ."
 - "set PATH=C:\\Ruby25-x64\\bin;%PATH%"
-- "gem install bundler --no-ri --no-rdoc"
+- "gem --version"
+- "gem install bundler -v 1.17.3 --no-ri --no-rdoc"
 - "bundler --version"
 
 test_script: