1.26.4: specify version of clean-css to address ReDOS vulnerability (https://…
…github.com/jakubpawlowicz/clean-css/commit/2929bafbf8cdf7dccb24e0949c70833764fa87e3)
…github.com/jakubpawlowicz/clean-css/commit/2929bafbf8cdf7dccb24e0949c70833764fa87e3)