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>
Merge pull request #336 from Koronen/remove-foreman-from-gemfile

Remove foreman from Gemfile
Clean up Gemfile

* Sort gem declarations lexicographically.
* Use `arel` from RubyGems.org.
* Add pessimistic version lock on `rack-ssl`.

Fixes #334.
Merge pull request #338 from Koronen/clean-up-gemfile

Clean up Gemfile
Upgrade rack-protection to workaround issues with recent Chrome updates

Fixes https://github.com/swanson/stringer/issues/335
Merge pull request #339 from pascalw/bugfix-335-rack-protection

Upgrade rack-protection to workaround issues with recent Chrome updates
Merge remote-tracking branch 'upstream/master'