Use latest build for samples, run without opengl

`require` latest build rather than latest installed version. This makes
it easier to test out samples for a new build without affecting the
currently installed rbSFML.

If opengl gem isn't installed, catch the LoadError and print an error,
so `rake samples` will still run.
Slight comment/whitespace tweak

The ruby-opengl gem is now just opengl. See
https://github.com/larskanis/ruby-opengl
Merge pull request #44 from silverhammermba/fix_samples

Small Fixes for Samples