Upgrade to vcr ~> 2.4 to prevent KeyError with Ruby 2.0

```rake test```
```
  9) Error:
test_0008_returns en language for symbol(WtfLang):
KeyError: key not found: "recorded_at"
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/vcr-2.9.2/lib/vcr/structs.rb:511:in `fetch'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/vcr-2.9.2/lib/vcr/structs.rb:511:in `from_hash'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/vcr-2.9.2/lib/vcr/cassette.rb:175:in `block in previously_recorded_interactions'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/vcr-2.9.2/lib/vcr/cassette.rb:175:in `map'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/vcr-2.9.2/lib/vcr/cassette.rb:175:in `previously_recorded_interactions'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/vcr-2.9.2/lib/vcr/cassette.rb:75:in `http_interactions'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/vcr-2.9.2/lib/vcr.rb:289:in `http_interactions'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/vcr-2.9.2/lib/vcr/request_handler.rb:75:in `stubbed_response'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/vcr-2.9.2/lib/vcr/request_handler.rb:68:in `has_response_stub?'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/vcr-2.9.2/lib/vcr/request_handler.rb:37:in `request_type'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/vcr-2.9.2/lib/vcr/request_handler.rb:10:in `handle'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/vcr-2.9.2/lib/vcr/library_hooks/webmock.rb:127:in `block in <module:WebMock>'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/webmock-1.18.0/lib/webmock/stub_registry.rb:28:in `call'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/webmock-1.18.0/lib/webmock/stub_registry.rb:28:in `block in register_global_stub'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/webmock-1.18.0/lib/webmock/request_pattern.rb:35:in `call'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/webmock-1.18.0/lib/webmock/request_pattern.rb:35:in `matches?'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/webmock-1.18.0/lib/webmock/stub_registry.rb:58:in `block in request_stub_for'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/webmock-1.18.0/lib/webmock/stub_registry.rb:57:in `each'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/webmock-1.18.0/lib/webmock/stub_registry.rb:57:in `detect'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/webmock-1.18.0/lib/webmock/stub_registry.rb:57:in `request_stub_for'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/webmock-1.18.0/lib/webmock/stub_registry.rb:50:in `response_for_request'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/webmock-1.18.0/lib/webmock/http_lib_adapters/net_http.rb:79:in `request'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/net/http.rb:1278:in `request_get'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/net/http.rb:479:in `block in get_response'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/webmock-1.18.0/lib/webmock/http_lib_adapters/net_http.rb:123:in `start_without_connect'
    /home/jason/.rvm/gems/ruby-2.0.0-p247@wtf_lang/gems/webmock-1.18.0/lib/webmock/http_lib_adapters/net_http.rb:150:in `start'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/net/http.rb:582:in `start'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/net/http.rb:477:in `get_response'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/net/http.rb:454:in `get'
    /www/wtf_lang/lib/wtf_lang/api.rb:19:in `send'
    /www/wtf_lang/lib/wtf_lang/api.rb:27:in `detect'
    /www/wtf_lang/lib/wtf_lang/core_ext/extension.rb:8:in `detect_lang'
    /www/wtf_lang/lib/wtf_lang/core_ext/extension.rb:12:in `lang'
    /www/wtf_lang/test/wtf_lang_test.rb:53:in `block (2 levels) in <top (required)>'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/minitest/unit.rb:1301:in `run'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/minitest/unit.rb:919:in `block in _run_suite'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/minitest/unit.rb:912:in `map'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/minitest/unit.rb:912:in `_run_suite'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/minitest/unit.rb:899:in `block in _run_suites'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/minitest/unit.rb:899:in `map'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/minitest/unit.rb:899:in `_run_suites'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/minitest/unit.rb:867:in `_run_anything'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/minitest/unit.rb:1060:in `run_tests'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/minitest/unit.rb:1047:in `block in _run'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/minitest/unit.rb:1046:in `each'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/minitest/unit.rb:1046:in `_run'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/minitest/unit.rb:1035:in `run'
    /home/jason/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/minitest/unit.rb:789:in `block in autorun'
```

Per https://github.com/vcr/vcr/issues/270, upgrade to vcr 2.4 and reset cassettes fixes.