remove default value for the data-graph-yaxis-X-title-text attribute

The default value for the data-graph-yaxis-X-title-text attribute is "Valeur".

It should'nt be a french value. We can remove the default value (see #8).

People using the default value can do something like this for backward compatibility : 

```js
$('table.highchart').bind('highchartTable.beforeRender', function(event, highChartConfig) {
  $.each(highChartConfig.yAxis, function (index, value)   {
    value.title.text = value.title.text || 'Valeur';
  });
});
```