Merge pull request #776 from murilocadanus/master

FIX: Changed project reference link
Removed less frequently used libraries

Closes https://github.com/PistonDevelopers/piston/issues/777
Merge pull request #778 from bvssvni/master

Removed less frequently used libraries
Made Gfx opt-in

Closes https://github.com/PistonDevelopers/piston/issues/784
Merge pull request #785 from bvssvni/master

Made Gfx opt-in
Pointing sdl2 to crates.io
Merge pull request #788 from bvssvni/master

Pointing sdl2 to crates.io
Upgraded to latest piston-current
Merge pull request #790 from bvssvni/master

Upgraded to latest piston-current
Fixed Travis CI
Merge pull request #791 from bvssvni/master

Fixed Travis CI
Added `DANGER` to `render_2d_opengl` and `render_2d_gfx`
Merge pull request #792 from bvssvni/master

Added `DANGER` to `render_2d_opengl` and `render_2d_gfx`
Reexported `DANGER`
Merge pull request #793 from bvssvni/master

Reexported `DANGER`