Remove foreman from Gemfile

> You should not put `foreman` into your `Gemfile` as the dependencies of a developer utility should not be able to interfere with the dependencies of your app.

<https://github.com/ddollar/foreman/pull/437#issuecomment-41110407>

> Ruby users should take care *not* to install foreman in their project's `Gemfile`.

<https://github.com/ddollar/foreman/blob/v0.68.0/README.md#installation>