Fixes long bundler resolution times when there is no pre-existing Gemfile.lock

This should fix #1382.

Bundler is falling into a long-running dependency lookup trap when trying to resolve mime-types and builder versions that work with the other dependencies. It will eventually finish, but I gave up after about 15 minutes of allowing it to run on my machine.

I would have liked to add the hinting to the gemspec. However you can't have both a development and runtime dependency with the same name defined.

## Repro Steps
```
paperclip $ rm Gemfile.lock; bundle
Fetching gem metadata from https://rubygems.org/......
Fetching additional metadata from https://rubygems.org/..
Resolving dependencies.................................. # Will take an exceptionally long time...
```