Force installation Bundler

AppVeyor says

gem install bundler --no-document
bundler's executable "bundle" conflicts with C:/Ruby200/bin/bundle
Overwrite the executable? [yN]

and it timed out. This commit adds -f option for it.

appveyor.yml

@@ -2,7 +2,7 @@
 clone_depth: 10
 install:
   - SET PATH=C:\Ruby%ruby_version%\bin;%PATH%
-  - gem install bundler --no-document
+  - gem install bundler --no-document -f
   - bundle install
 build: off
 test_script: