Percentile aggregator fails on my maching due to issues converting double.

My locale expects doubles to be in the 1,0 format, so the double.parse
fails if it tries to parse 1.0.
Added CultureInfo.InvariantCulture to the parse to fix this.
Merge pull request #793 from andersosthus/percentile-decimal-fix

Percentile aggregator fails on my maching due to error converting double