Stop using deprecated RefCell<T>::get

It's going away in the next Rust upgrade.
auto merge of #2135 : mbrubeck/servo/refcell-get, r=larsbergstrom

It's going away in the next Rust upgrade (mozilla/rust#13301). r? @larsbergstrom

We'll need to do the same in rust-layers.
Add instructions for building servo on Fedora Core.
auto merge of #2138 : cgjones/servo/fedora-build-instructions, r=jdm

Pretty straightforward, except that Fedora doesn't provide a package with the MS core fonts, so devs have to build the package themselves.