Update to stable doesn't select new version
```
~ » choosenim update stable
   Updating stable
      Info: Already up to date at version 0.17.0
~ » choosenim show         
   Channel: stable
   Version: 0.16.0
      Path: /Users/dom/.choosenim/toolchains/nim-0.16.0
~ » choosenim -v                                                                                                               1 ↵
choosenim v0.2.2 (2017-05-17 22:47:01) [macosx/amd64]
```