Add plugin = true to gl_generator
Update glfw-rs.

Reference: https://github.com/bjz/glfw-rs/pull/203.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Merge pull request #118 from omasanori/master

Update glfw-rs.
Merge pull request #117 from tomaka/plugin

plugin = true for gl_generator