Install libgl1-mesa-dev when testing with Travis CI
Install libgl1-mesa-dev when testing with Travis CI

Compiling SDL2_ttf on Ubuntu in Travis CI requires libGL.so, which is
included in libgl1-mesa-dev. Refer to
https://discourse.libsdl.org/t/cross-compiling-sdl-ttf/18773.

Cobrand/rust-sdl2,Rust-SDL2/rust-sdl2,Cobrand/rust-sdl2,AngryLawyer/rust-sdl2,Rust-SDL2/rust-sdl2,Cobrand/rust-sdl2,Rust-SDL2/rust-sdl2