Fix page.rb content method

When pulling Wikipedia content "text/x-wiki" is returned. I'm using ruby 1.9.3p385 with rvm.
```
require 'wikipedia'
 => true

Wikipedia.find("Salesian High School").content
 => "text/x-wiki"
```

Fixed by changing page.rb to lookup key directly from hash.
```
page['revisions'].first["*"]
```

Now it responds with
```
require 'wikipedia'
 => true

 Wikipedia.find("Salesian High School").content
 => "'''Salesian School''' may refer to:\n\n* [[Salesian schools]],
a generic term applied to educational institutions run by the Roman
Catholic Salesian Congregation of Saint John Bosco\n....
```