Use the same User-Agent in every API call.

TBH I don't know how to test it.

VCR lets you match on request headers (https://www.relishapp.com/vcr/vcr/v/2-5-0/docs/request-matching/matching-on-headers) with 

```ruby
VCR.use_cassette('alice-gets-bob', :match_requests_on => [:headers]) do
...
end
```

But then you have to modify the VCR cassettes everytime you bump exercism's version.

Any ideas? Do we need to test the user agent here?