pass a parameter to `box-flex`

With no parameter on `box-flex` the resulting -webkit-box-flex and similar
parameters have no value, which does ot validate as proper CSS.

This is a potential fix for #34
Merge pull request #44 from copiousfreetime/bugfix/box-flex

Possible fix for #34, pass a parameter to `box-flex`
Merge pull request #1 from soyjavi/master

pass a parameter to `box-flex`