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