Stopped using constants for default host values

This fixes https://github.com/reset/github-cookbook/issues/4

Declaring constants in library files throws warning messages when they are "redefined" with each use of the file. I don't see any reason for these values to be constants.

```
/libraries/github_archive.rb:15: warning: already initialized constant DEFAULT_HOST
/libraries/github_asset.rb:26: warning: already initialized constant DEFAULT_HOST
```