Remove workaround in Appveyor's configuration

The issue is now fixed upstream, so go back to the old config.

Author: David Rodríguez

travis/appveyor.yml

@@ -15,7 +15,7 @@ install:
   - SET PATH=C:\Ruby%ruby_version%\bin;%PATH%
   - ruby --version
   - gem --version
-  - gem install bundler -v 1.11.2 # pin 1.11.2 version until later versions work
+  - gem install bundler
   - bundler --version
   - bundle install
   - cinst ansicon