Update timecop requirement from ~> 0.8.0 to ~> 0.9.1

Updates the requirements on [timecop](https://github.com/travisjeffery/timecop) to permit the latest version.
- [Release notes](https://github.com/travisjeffery/timecop/releases)
- [Changelog](https://github.com/travisjeffery/timecop/blob/master/History.md)
- [Commits](https://github.com/travisjeffery/timecop/commits/v0.9.1)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

sensu-plugins-ssl.gemspec

@@ -43,6 +43,6 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'redcarpet',                 '~> 3.2'
   s.add_development_dependency 'rspec',                     '~> 3.1'
   s.add_development_dependency 'rubocop',                   '~> 0.89.1'
-  s.add_development_dependency 'timecop',                   '~> 0.8.0'
+  s.add_development_dependency 'timecop',                   '~> 0.9.1'
   s.add_development_dependency 'yard',                      '~> 0.8'
 end