Remove Math.sign

[alexcjohnson]

Math.sign is not supported in IE, even IE11. Currently we only seem to have this in gl2d/camera
https://github.com/plotly/plotly.js/blob/master/src/plots/gl2d/camera.js#L134
https://github.com/plotly/plotly.js/blob/master/src/plots/gl2d/camera.js#L152

We should remove these and include Math.sign (any other IE quirks?) in our syntax tests.

[alexcjohnson]

fixed by #1792