Update omniauth requirement from ~> 1.0 to >= 1, < 3

Updates the requirements on [omniauth](https://github.com/omniauth/omniauth) to permit the latest version.
- [Release notes](https://github.com/omniauth/omniauth/releases)
- [Commits](omniauth/omniauth@v1.9.1...v2.1.0)

---
updated-dependencies:
- dependency-name: omniauth
  dependency-type: direct:production
...

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

Gemfile.lock

@@ -2,16 +2,19 @@ PATH
   remote: .
   specs:
     omniauth-ucam-raven (1.0.1)
-      omniauth (~> 1.0)
+      omniauth (>= 1, < 3)
 
 GEM
   remote: https://rubygems.org/
   specs:
-    hashie (4.1.0)
-    omniauth (1.9.1)
+    hashie (5.0.0)
+    omniauth (2.1.0)
       hashie (>= 3.4.6)
-      rack (>= 1.6.2, < 3)
-    rack (2.2.3.1)
+      rack (>= 2.2.3)
+      rack-protection
+    rack (3.0.0)
+    rack-protection (2.2.2)
+      rack
 
 PLATFORMS
   ruby

omniauth-ucam-raven.gemspec

@@ -17,6 +17,6 @@ Gem::Specification.new do |s|
   s.test_files    = s.files.grep(%r{^(test|spec|features)/})
   s.require_paths = ['lib']
 
-  s.add_dependency 'omniauth', '~> 1.0'
+  s.add_dependency 'omniauth', '>= 1', '< 3'
   s.add_development_dependency 'bundler', '~> 2.0'
 end