Fixes incorrect calculation of Specific Gravity from provided Gravity Units

Something wasn't looking quite right when I dove in here, and was a little thrown off. I [grabbed another calculator](http://pint.com.au/calculators/gravity/) to be sure my original thoughts were correct and it appears they were.

I'd write up some tests, but I'm doing this PR from the github editor.
Merge pull request #166 from csaunders/patch-1

Fixes incorrect calculation of Specific Gravity from provided GU