axis.min and axis.max don't accept decimal numbers

[jeromecyang]

https://github.com/jettro/c3-angular-directive/blob/master/src/axis-y-directive.js#L103
https://github.com/jettro/c3-angular-directive/blob/master/src/axis-y-directive.js#L106

Since parseInt is used, any decimal place is removed when parsed.