Fix contain check in prefix-value

If I add `prefixValue("order")`, `box-sizing: border-box` will be compiled to `box-sizing: b-moz-order-box`, because Rework try to find any `order` symbols in CSS value.

This patch fix CSS value check and `prefixValue` will try to find word, not any letters like `order`.

This fix need to close bug in Autoprefixer: https://github.com/ai/autoprefixer/issues/3