Fix line for Gemfile.

Hi!

I was about to install this gem, and I ran into a problem while adding the gem to my gemfile. It seems using the line on the README:

```ruby
    gem 'immobilienscout24', :git => 'git@github.com:endil/immobilienscout24.git'
```

only works if you have permission to access the repo through ssh. If you do it using the read-only link (as done in the examples [here](http://gembundler.com/git.html)) it works:

```ruby
    gem 'immobilienscout24', :git => 'git://github.com/endil/immobilienscout24.git'
```

In this PR you can see the updated README.md showing this.

Thanks for reading!