Update LanguageDeserializer.cs after 5b89187f6ce9018a8497d905ebb3869ef04878f4.
Fix test deployment.
Add a custom test script to appveyor.yml.
Various AppVeyor updates:

1. fix `CONFIGURATION` environment variable
2. set verbosity to minimal and specify the solution file
3. enable build cache
4. enable fast_finish

Close #275 by merging it.