Change the fixture helper

Now allows you to specify type of fixture (defaults to json)
Adds typical Cloudflare error fixture

As seen by @Ferada:

```
0 ~ % tugboat ssh -e my-host
Droplet fuzzy name provided. Finding droplet ID...757: unexpected token at '<!DOCTYPE html>
<html>
<head>
  <title>DigitalOcean - Seems we've encountered a problem!</title>
  <style type="text/css">
    body {
```
New spec to catch when non-JSON errors given

Spec now fails as intended:

```
 Failure/Error: expect { @cli.verify }.to raise_error(SystemExit)
       expected SystemExit, got #<NoMethodError: undefined method `status' for #<String:0x007f80e3a2c590>> with backtrace:
         # ./lib/tugboat/middleware/authentication_middleware.rb:24:in `rescue in call'
         # ./lib/tugboat/middleware/authentication_middleware.rb:20:in `call'
         # ./lib/tugboat/middleware/check_credentials.rb:11:in `call'
         # ./lib/tugboat/middleware/inject_client.rb:33:in `call'
         # ./lib/tugboat/middleware/check_configuration.rb:19:in `call'
         # ./lib/tugboat/middleware/inject_configuration.rb:10:in `call'
         # ./lib/tugboat/cli.rb:49:in `verify'
         # ./spec/cli/verify_cli_spec.rb:26:in `block (4 levels) in <top (required)>'
         # ./spec/cli/verify_cli_spec.rb:26:in `block (3 levels) in <top (required)>'
```
Add a guard around checking if response is Hashie

We should only be returning the hash error if the response given is a Hashie::Rash. Otherwise it's probably a non-JSON response and will just be a string
Add a 401 guard to asking someone to check creds

No point asking a user to check credentials if it gives a 500 error
Only parse JSON if content-type is json

Use regex to check content type given is json
Ensure webmock gives correct content-type

Otherwise Faraday won't parse mocks with json middleware
Ensure html is given as html content-type
Fix string match for HTML errors
Remove all empty header setting
Merge pull request #139 from pearkes/handle_non_json_errors

Handle HTML Error responses