Test more recent versions of Ruby on Travis

Added:
- 2.4.9 (reached end-of-life but may temporarily help in incremental debugging)
- 2.5.8
- 2.6.6

Did not add 2.7.x yet because it introduces *a lot* of warnings.

Did not remove any past version yet because until decided otherwise,
they should be supported by the gem. (I would recommend a major version
bump when those versions get dropped.)

The idea is that developments from now on should be future-proof, and
changes should be tested on current versions of Ruby.

Author: davidstosik

.travis.yml

@@ -4,6 +4,9 @@ rvm:
   - 2.1.8
   - 2.2.4
   - 2.3.0
+  - 2.4.9
+  - 2.5.8
+  - 2.6.6
 gemfile:
   - Gemfile
 script:

rspec_api_documentation.gemspec

@@ -26,7 +26,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency "rake", "~> 10.5.0"
   s.add_development_dependency "rack-test", "~> 0.6.3"
   s.add_development_dependency "rack-oauth2", "~> 1.2.2"
-  s.add_development_dependency "webmock", "~> 1.22.6"
+  s.add_development_dependency "webmock"
   s.add_development_dependency "rspec-its", "~> 1.2.0"
   s.add_development_dependency "faraday", "~> 0.9.2"
   s.add_development_dependency "nokogiri", "~> 1.8.4"