ActionPack has removed `deep_munge` and this gem no longer works with
Rails 5.

This bumps the dependency for ActionPack which requires Rack 2.0 so I
added that to the Gemfile as well since it's unreleased.

`deep_munge` no longer exists. We should now use
`normalize_encode_params`

Following rails/rails#20868 changed how
ActionController::Parameters worked.

This change makes `dump_param_keys` match how the Rails side reads teh
hash keys.

Without this change the right keys don't make it to `Request::Utils` so
the test `test_dasherized_keys_as_xml` was failing because `sub-key`
wasn't being recognized as a key.

Passing middleware as strings or symbols is now deprecated.

```
DEPRECATION WARNING: Passing strings or symbols to the middleware
builder is deprecated, please change
them to actual class references.  For example:

  "ActionDispatch::XmlParamsParser" => ActionDispatch::XmlParamsParser
```

It is now required that HTTP request methods pass keyword arguments
instead of params without keywords.

This fixes the following deprecation warning:

```
DEPRECATION WARNING: ActionDispatch::IntegrationTest HTTP request
methods will accept only the following keyword arguments in future
Rails versions:
params, headers, env, xhr

Examples:

get '/profile',
  params: { id: 1 },
  headers: { 'X-Extra-Header' => '123' },
  env: { 'action_dispatch.custom' => 'custom' },
  xhr: true
```

Fixes deprecation warning to swicth from using `render text:` to `render
plain:`.

```
`render :text` is deprecated because it does not actually render a
`text/plain` response. Switch to `render plain: 'plain text'` to render
as `text/plain`
```

Rails 5 requires Ruby 2.2.2 and will fail on versions lower than 2.2.2.
This change removes the versions that cannot work with the changes to
Rails 5 and actionpack-xml_parser. Updates corresponding .travis.yml
file so that master no longer runs against those.