How can I make SwiftKey the default on iOS 8?
Settings>General>Keyboard>Keyboards sets the keyboards you wish to use. The top keyboard in the list is the Default. "Edit" will allow you to edit, remove keyboards or move a keyboard to the top of the list.  To get Swiftkey as the default keyboard, move it to the top of the list.  I tried it and it works for me, of course as per below there are instances when Swiftkey wont be the default keyboard.

However, there are instances where your default keyboard will NOT be used, no matter the settings. Examples include when security settings, passwords etc are required, the standard Apple keyboard (now referred to as 'Quicktype') will launch. So do not expect the 3rd party keyboard to function exactly as the built-in one does...there will be places where it refuses to launch, usually due to a security issue.

As of Sept 2014, this is a new feature, so it is likely that there are refinements that will happen over time to help make the use of 3rd party keyboards more seamless.