Support latest GSL release

[v0dro]

GSL v2.1 has just been released. The current rb-gsl is not working with it, which it should ( #24 ).

Question for @blackwinter @minad : Should we maintain backwards compatibility for pre-2.1 GSL versions or force the user to upgrade?

I'd say no need for backward compatibility. What are your views?

[minad]

we already dropped ancient versions. however I think 1.16 is still widely deployed

[rbalint]

Debian has migrated to GSL 2.x in unstable making ruby-gsl package unusable due to this bug. :-(
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=804499

[rbalint]

Without GSL2.x support ruby-gsl won't be released as part of next Debian nor Ubuntu stable. :-(
Installing the gem on Debian and Ubuntu will be also harder since they won't ship GSL 1.16.

[v0dro]

I'm on it. Will be releasing a new version with GSL 2.x support by March first/second week.

[v0dro]

#33