require mechanize library

I think , you may be want to require mechanize, because it needed in [fake_browser.rb](lib/vk-ruby/fake_browser.rb)  - VK::FakeBrowser#agent. Without it , when i trying:
```ruby
app = VK::Application.new(app_id: my_app_id)
client = app.client_auth(login: my_login, password: my_password)
```
I have the following error: 
```bash
VK::AuthentificationError: Authentification error: invalid loging or password
  from /Users/ruslan/fun/vk_fun/vk-ruby/lib/vk-ruby/fake_browser.rb:13:in `rescue in sign_in!'
```
So, through debugging, i noticed that Mechanize is not required => agent is not defined => Exception on 13 line(see above) is raised , that's why it doesn't work without it.
**It seems, that you required mechanize in [vk interactive shell](lib/vk-ruby/irb.rb), but I didn't use it** :wink: