msttcorefonts is a requirement

build and make check will pass, but assertion will fail if msttcorefonts is not installed.
auto merge of #605 : rfielding/servo/master, r=metajack

assertion will fail if msttcorefonts is not installed.  the page will come up successfuly now  when literally following the README.md (a segfault when the window is closed, but good progress now).