Allowing setting "title" from inside a method.

Updated to allow setting a title inside a method, e.g.:

```ruby
class HomeScreen < ProMotion::Screen
  
  title "Home Screen"
  
  def on_load
    title "Something dynamic"
  end
end
```

The title will be updated to *Something dynamic*.

Additionally, fixed a deprecation warning when running `bundle`, as `source :rubygems` is superseded by `source 'https://rubygems.org'.

Updated version to 0.4.2.