Update CSS Parser to eliminate CSS3 calc() workaround

[westonruter]

As a site admin, I should have the fix that eliminates CSS3 calc() workarounds inside the AMP core codebase.

• AC1: Merge in the code (as specified below) and validate the CSS parser works as expected, removing the lingering workaround in our plugin core.

---

In #1116 we added a workaround to preserve CSS3 calc() functions since they weren't supported in PHP-CSS-Parser. However, now they are: MyIntervals/PHP-CSS-Parser#128. So we can merge the latest into our fork and eliminate the calc() placeholders.

[postphotos]

Hi @westonruter, Thanks for this! I've written up a quick user story here. It sounds like this should be pretty straightforward to squeeze into our v1.0 stable release. 👍