Error when running in Safari 7: setLineDash results in a TypeError

Calling setLineDash results in a TypeError in Safari 7.0 (9537.51.3) in 10.9 DP4. None of the editor samples currently load completely in Safari 7 (e.g., the arc.json sample renders just a single pie wedge before erroring). The error in the Javascript console when loading the arc.json sample is below.

This pull request fixes the issue by assign ctx.lineDash directly instead of calling ctx.setLineDash(), which, for some reason, fixes the issue.

```
[Error] TypeError: Type error
	vgLineDash (vega.js, line 1862)
	drawPathOne (vega.js, line 1382)
	drawPathAll (vega.js, line 1395)
	(anonymous function) (vega.js, line 1554)
	draw (vega.js, line 1951)
	drawGroup (vega.js, line 1591)
	draw (vega.js, line 1951)
	render (vega.js, line 1930)
	render (vega.js, line 6549)
	update (vega.js, line 6578)
	(anonymous function) (editor.js, line 64)
	(anonymous function) (vega.js, line 4607)
```