Update rspec requirement from ~> 2.6.0 to ~> 3.12.0

Updates the requirements on [rspec](https://github.com/rspec/rspec-metagem) to permit the latest version.
- [Release notes](https://github.com/rspec/rspec-metagem/releases)
- [Commits](rspec/rspec-metagem@v2.6.0...v3.12.0)

---
updated-dependencies:
- dependency-name: rspec
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

omniauth-uaa-oauth2.gemspec

@@ -18,6 +18,6 @@ Gem::Specification.new do |gem|
   gem.add_runtime_dependency 'omniauth', '>= 1', '< 3'
   gem.add_runtime_dependency 'cf-uaa-lib', ['>= 3.2', '< 5.0']
 
-  gem.add_development_dependency 'rspec', '~> 2.6.0'
+  gem.add_development_dependency 'rspec', '~> 3.12.0'
   gem.add_development_dependency 'rake'
 end