SkillCalculator calls Exception in wrong namespace

This pull request solves a bug I have been victim of, where PHPSkills fails with the following error message:
```PHP Fatal error:  Class 'Moserware\\Skills\\Exception' not found in [redacted]/PHPSkills/Skills/SkillCalculator.php on line 73
```
Merge pull request #8 from jbruggem/patch-1

SkillCalculator calls Exception in wrong namespace