Changed dependency of sdl2 from git repo to cargo's index.
Changed --release to compile with --opt-level=2.
Renamed json::JsonArray to json::Array to reflect Rust's recent changes.
Added Copy deriving to characters::Direction.
Fixed lifetimes in game::Game.
Fixed some sdl2::video enums in graphics::Graphics to reflect recent rust-sdl2's changes.
Fixed utils::{Vector2,Vector2f,Rectangle}'s operators implementation after recent Rust's changes.
Allowed missing Copy implementation on enums and structs.
[TravisCI] Added LD_LIBRARY_PATH to env.