Forced square ratio

The latest release (2.2.0) sets the default ratio to ct-square, making it impossible to show charts where the height is scaling with the content.

https://github.com/Yopadd/vue-chartist/commit/205b20a61d8953863e2b7bf96e109b45f21a49fb#diff-1fdf421c05c1140f6d71444ea2b27638R12