Load the Faraday adapter.

This adds support for Faraday 0.9.x series, which don't automatically load all of its adapters.

before:
```
$ EXERCISM_ENV=test exercism fetch

ERROR: Something went wrong. Exiting.
uninitialized constant Faraday::Adapter::NetHttp

/Users/yaauie/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/exercism-0.0.20/lib/exercism/api.rb:13:in `block in conn': uninitialized constant Faraday::Adapter::NetHttp (NameError)
	from /Users/yaauie/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/faraday-0.9.0.rc5/lib/faraday/connection.rb:84:in `initialize'
	from /Users/yaauie/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/faraday-0.9.0.rc5/lib/faraday.rb:70:in `new'
	from /Users/yaauie/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/faraday-0.9.0.rc5/lib/faraday.rb:70:in `new'
	from /Users/yaauie/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/exercism-0.0.20/lib/exercism/api.rb:12:in `conn'
	from /Users/yaauie/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/exercism-0.0.20/lib/exercism/api.rb:45:in `get_and_save'
	from /Users/yaauie/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/exercism-0.0.20/lib/exercism/api.rb:23:in `fetch'
	from /Users/yaauie/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/exercism-0.0.20/lib/cli.rb:36:in `fetch'
	from /Users/yaauie/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/thor-0.18.1/lib/thor/command.rb:27:in `run'
	from /Users/yaauie/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/thor-0.18.1/lib/thor/invocation.rb:120:in `invoke_command'
	from /Users/yaauie/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/thor-0.18.1/lib/thor.rb:363:in `dispatch'
	from /Users/yaauie/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/thor-0.18.1/lib/thor/base.rb:439:in `start'
	from /Users/yaauie/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/exercism-0.0.20/bin/exercism:8:in `<top (required)>'
	from /Users/yaauie/.rbenv/versions/1.9.3-p194/bin/exercism:23:in `load'
	from /Users/yaauie/.rbenv/versions/1.9.3-p194/bin/exercism:23:in `<main>'
```

after:

```
$ EXERCISM_ENV=test exercism fetch
No assignments fetched.
```