[fix-build] Lock onto Thor version 0.19.1
[fix-build] Lock onto Thor version 0.19.1

Looks like 0.19.2 (released this morning) is broken:

http://stackoverflow.com/questions/40823526/validate-default-type-an-options-default-must-match-its-type-argumenterror

Without this change, I'm seeing this error when running specs:

```
  1) RevealCK::Commands::ListenToReloadBrowser#run works with Guard to setup livereload
     Failure/Error: require 'guard/cli'

     ArgumentError:
       An option's default must match its type.
```

jedcn/reveal-ck,jedcn/reveal-ck,jedcn/reveal-ck