contenttest: Exit with non-zero status when tests fail

This makes buildbot reject commits with failing contenttests.
mk: Update dependencies.
auto merge of #1130 : kmcallister/servo/contenttest, r=metajack

**This build is expected to fail** at first, because `test_img_width_height` is broken on `master`. It's the simplest way I know to check that with this change, buildbot will indeed catch failing content tests.

After that happens I'll push the commits to fix `test_img_width_height` and request a re-review.
auto merge of #1131 : pcwalton/servo/update-dependencies, r=metajack

r? @kmcallister
Fix computed value of the 'display' property.
Use Au::scale_by instead of duplicating it.
Fix selector parsing.
Move CSS21 pseudo-elements parsing out of parse_simple_pseudo_class
Simplify code that is now redundant with the computed value of 'display'.
Implementation of pseudo class 'first-child'.