Refine calculation of Code Refactoring Ratio to handle ratios under 1.0

[amclin]

The calculations of Code Refactoring Ratio needs to be revised because it is non-continugous:

1. A ratio of undefined would be considered good ( code changes divided by 0 test changes)
2. A ratio of < 1 would be considered bad (more test changes than code changes), and the close to 0 that it gets, the worse it is
3. Ratios above 1.0 increase positively as the value increases

https://github.com/amclin/devopsculture/blob/master/_kpis/code-refactoring.md#todo-future-refinement

With these definitions, a Refactoring Ratio of undefined is ideal (lots of code changes, with no test changes). But a fractional Refactoring Ratio value between 0 and 1 is bad as it indicates code changes require significantly more testing.