ruby2.1-dev: Depend on libgmp-dev

Thanks: John Leach <john@johnleach.co.uk>
Fix FTBFS with libreadline 6.x

Applied upstream r45225.
Prepare changelog for 2.1.0-2
Imported Upstream version 2.1.1
Merge tag 'upstream/2.1.1' into master-2.1

Upstream version 2.1.1
Document example for generating orig source

Git-Dch: ignore
Update lintian overrides
Prepare changelog for 2.1.1-1
Tie Tcl/Tk dependency to version 8.5

Upstream doesn't support 8.6 yet.
Prepare changelog for 2.1.1-2
Tie Tcl/Tk dependency to version 8.5

Upstream doesn't support 8.6 yet.

Thanks: Matthias Klose <doko@debian.org>
Prepare changelog for the real 2.1.1-2 release
Merge remote-tracking branch 'debian/master-2.1'

Conflicts:
	debian/changelog
	ext/bigdecimal/bigdecimal.gemspec
Add alternatives back in

Debian is no longer allowing switching between ruby versions using the
alternatives system. We want to continue supporting it (at least in
Ubuntu versions where it is already supported).

Fixes GH #3
Add dependency on rubygems-integration
Fix deps for tk building on pre-trusty
ruby2.1 (2.1.1-2bbox3)