If a manifest file was detected skip rake detect

When a user is deploying with a manifest, we don't not need to load up their Rake to see if they have the `asset:precompile` task. Changing the order of the checks prevents users from seeing debugging information that is not needed and helps speed up apps by doing less work.
Merge pull request #188 from heroku/schneems/no-rake-if-manifest

If a manifest file was detected skip rake detect