add Gem::Specification#bindir to the template of gemspec so that user can change their executables dir easily

I sometimes change executables dir from bin to exe.
I opened gemspec and edited "spec.executables", but after releasing my gem I found executables installed did not work.

That is because installed executables loads file under bindir, but bindir is still bin.

I should also edit not only spec.executables but also "spec.bindir".

When default gemspec have the Gem::Specification#bindir and Gem::Specification#executables uses it, No mistake like me will happen.

Regards,
okitan