Removed hgl-rs triangle example

Moved to https://github.com/PistonDevelopers/piston-example-hgl_triangle

Closes https://github.com/PistonDevelopers/piston/issues/436
Removed hgl-rs triangle example

Moved to https://github.com/PistonDevelopers/piston-example-hgl_triangle

Closes https://github.com/PistonDevelopers/piston/issues/436
Merge branch 'master' of https://github.com/bvssvni/piston
Merge pull request #437 from bvssvni/master

Removed hgl-rs triangle example