Update activesupport requirement from ~> 6.0.0 to >= 6.0, < 6.2 (#49)

Updates the requirements on [activesupport](https://github.com/rails/rails) to permit the latest version.
- [Release notes](https://github.com/rails/rails/releases)
- [Changelog](https://github.com/rails/rails/blob/v6.1.0/activesupport/CHANGELOG.md)
- [Commits](rails/rails@v6.0.0...v6.1.0)

Signed-off-by: @marcqualie

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

diffcrypt.gemspec

@@ -29,6 +29,6 @@ Gem::Specification.new do |spec|
   spec.executables = %w[diffcrypt]
   spec.require_paths = ['lib']
 
-  spec.add_runtime_dependency 'activesupport', '~> 6.0.0'
+  spec.add_runtime_dependency 'activesupport', '>= 6.0', '< 6.2'
   spec.add_runtime_dependency 'thor', '>= 0.20', '< 2'
 end