Prefer https over http.

When using the old format Berksfile with Berkshelf 3.x, the following deprecation message is displayed.
```
DEPRECATED: Your Berksfile contains a site location pointing to the Opscode Community Site (site :opscode). Site locations have been replaced by the source location. Change this to: 'source "http://api.berkshelf.com"' to remove this warning. For more information visit https://github.com/berkshelf/berkshelf/wiki/deprecated-locations
```

This change just modified the message so that it encourages the use of `https` over `http`.