Bump the minimum supported Ruby version from 1.9 to 2.3 (#208)

.rubocop.yml

@@ -9,3 +9,6 @@ Metrics/ClassLength:
   Max: 110
 Metrics/ModuleLength:
   Max: 110
+Metrics/BlockLength:
+  Exclude:
+    - "googleauth.gemspec"

README.md

@@ -184,7 +184,7 @@ Custom storage implementations can also be used. See
 
 ## Supported Ruby Versions
 
-This library is currently supported on Ruby 1.9+.
+This library is currently supported on Ruby 2.3+.
 
 However, Ruby 2.4 or later is strongly recommended, as earlier releases have
 reached or are nearing end-of-life. After March 31, 2019, Google will provide

googleauth.gemspec

@@ -25,6 +25,7 @@ Gem::Specification.new do |gem|
   end
   gem.require_paths = ["lib"]
   gem.platform      = Gem::Platform::RUBY
+  gem.required_ruby_version = ">= 2.3.0"
 
   gem.add_dependency "faraday", "~> 0.12"
   gem.add_dependency "jwt", ">= 1.4", "< 3.0"