RuboCop 0.52.0 linting (#603)

* gemspec: Make 2.2.2 the earliest-supported Ruby version

* Exclude linting which came out wrong

.rubocop.yml

@@ -80,3 +80,10 @@ Metrics/BlockLength:
 Lint/InterpolationCheck:
   Enabled: false
 
+Style/FormatStringToken:
+  Exclude:
+    - lib/github_changelog_generator/parser.rb
+
+Style/MixinUsage:
+  Exclude:
+    - lib/github_changelog_generator/task.rb

github_changelog_generator.gemspec

@@ -9,7 +9,7 @@ Gem::Specification.new do |spec|
   spec.version            = GitHubChangelogGenerator::VERSION
   spec.default_executable = "github_changelog_generator"
 
-  spec.required_ruby_version = ">= 1.9.3"
+  spec.required_ruby_version = ">= 2.2.2"
   spec.authors = ["Petr Korolev", "Olle Jonsson"]
   spec.email = "sky4winder+github_changelog_generator@gmail.com"