Fixed check for numeric value as proposed in issue #25
Fixed check for numeric value as proposed in issue #25

Added check for numeric value according to @zmogas patch in issue #25.

angular-ui/ui-slider,anoopelias/ui-slider,anoopelias/ui-slider,pallayr/ui-slider,brydar/ui-slider,ym/ui-slider,angular-ui/ui-slider,douglasduteil/ui-slider,asetiyadi/ui-slider,brydar/ui-slider,ym/ui-slider,pallayr/ui-slider,asetiyadi/ui-slider,kongx73/ui-slider,kongx73/ui-slider