Fix for scoping issue

As I [described in the Issues](https://github.com/donskifarrell/meteor-chartjs/issues/1), there was a problem with the new scoping scheme in Meteor 0.6.0. I found removing the `var` keyword in chart.min.js scopes the Chart variable to be global again. Afraid you might need to do that manually if upgrading to a newer version from upstream in future :(