Exclude pre-releases from release notes

## Expected Behaviour
Derek should compare new releases against the latest official release rather than against the latest release which can be a pre-release.

## Current Behaviour
When Derek generates the release notes he compares the last two releases regardless of whether they are pre-releases or not. This makes hard to read the changes from two official releases if there has been some release candidates.

## Possible Solution
Compare new releases against the latest official release.

## Steps to Reproduce (for bugs)
1. Create an official release (1.0.0)
2. Create a pre-release (1.1.0-rc.1)
3. Create an official release (1.1.0)
4. The generated notes will have no changes because its being compared against 1.1.0-rc.1 instead of 1.0.0

## Context
I was trying to create a new release from a release candidate but now it's hard to read the release notes of the major release because if I want to read all the changes I have to go through all pre-releases to know what have change