Proper check for currentMin/currentMax before prepending functions

#### Initial Problem
i load new data series dynamically over an api and display them using `highcharts-ng`. when a new series contains values which are not contained in the currently displayed axis range, it get not displayed.

i tracked this problem back to the statements https://github.com/pablojim/highcharts-ng/blob/master/src/highcharts-ng.js#L89 (and following fews). when commenting them, everything worked fine.

i noticed then, that `highcharts-ng` prepends these function as soon as a `axis` config is recognized in the chart config, whether `currentMax` or `currentMin` is present or not

#### About the Pull Request
this pull request improves the if statement wrapped around the prepending-code. with this, the presence of `currentMin` and/or `currentMax` is checked properly.

it may not fix the underlying problem completly, but it helps a little in the meantime :)