Require Ruby version >= 1.9.3 in gemspec.

It appears that the `rugged` gem only works with these versions of Ruby.

Although a Ruby version constraint has been added to the `rugged` gemspec,
this change has not yet been released (as of v0.19.0).

See the following:-

* https://github.com/libgit2/rugged/blob/development/rugged.gemspec#L20
* https://github.com/libgit2/rugged/pull/235
* https://github.com/libgit2/rugged/issues/236
* https://github.com/libgit2/rugged/commit/504a5c9b0ca00fbcde0007977a847f8e7a3618e3