Travis: Test w/Ruby 2.4 and 2.5

Requires updating webmock; see: bblimke/webmock#683

Author: danschmidt5189

.travis.yml

@@ -2,6 +2,8 @@ rvm:
   - 2.1
   - 2.2
   - 2.3.1
+  - 2.4
+  - 2.5
 branches:
   only:
     - master

omniauth-cas.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |gem|
   gem.add_dependency 'addressable',             '~> 2.3'
 
   gem.add_development_dependency 'rake',        '~> 10.0'
-  gem.add_development_dependency 'webmock',     '~> 1.19.0'
+  gem.add_development_dependency 'webmock',     '~> 2.3.1'
   gem.add_development_dependency 'rspec',       '~> 3.1.0'
   gem.add_development_dependency 'rack-test',   '~> 0.6'