Prepare Clearance for Rails 4

* Change routes to use `get` instead of `match`.
* Don't allow multiline strings for email format.
* Rails 4 issues an exception when validates_format_of encounters regex
  that uses ^ and $ rather than /A and /Z. See this pull request:
  https://github.com/rails/rails/pull/6569
* `sign_out` should use DELETE, not GET.
* Remove irrelevant spec for testing regex.
* Test against latest security-patched Rails versions.