Use downloaded snapshots instead of in-tree rust. Also a working linux travis conf
add libffi-dev
Merge pull request #1 from larsbergstrom/manish_snapshots

Add libffi-dev
auto merge of #2639 : Manishearth/servo/snapshots, r=larsbergstrom

`$CFG_TREE_RUST` brings back the old behavior

This also upgrades Rust (The new Rust works without any changes).