Don't load Vlad when loading the Rakefile.

This prevents two problems:

1. Configuring Vlad at Rakefile load time causes rake devenv invocation
   to fail: environment variables required for deployment (e.g. DOMAIN)
   cannot be expected to exist when starting up the development
   environment.

2. Something inside Vlad is defining a top-level source method, which
   interferes with irb applications (i.e. the Rails console).
Split up file and environment-based configuration.