Update supported ruby versions in CI

.travis.yml

@@ -2,12 +2,10 @@ language: ruby
 sudo: false
 cache: bundler
 rvm:
-  - 2.0.0
-  - 2.1.10
-  - 2.2.9
-  - 2.3.6
-  - 2.4.3
-  - 2.5.0
+  - 2.3.8
+  - 2.4.5
+  - 2.5.3
+  - 2.6.0
   - ruby-head
 matrix:
   include:
@@ -24,12 +22,7 @@ matrix:
     - rvm: ruby-head
     - rvm: rbx-3
     - rvm: jruby-head
-    - rvm: jruby
 before_install:
-  - "if $(ruby -e 'exit(RUBY_VERSION >= \"2.3.0\")'); then gem update --system; fi"
-  - "if $(ruby -e 'exit(RUBY_VERSION <  \"2.3.0\")'); then gem update --system 2.7.8; fi"
-  - "if $(ruby -e 'exit(RUBY_VERSION >= \"2.3.0\")'); then gem install bundler; fi" 
-  - "if $(ruby -e 'exit(RUBY_VERSION <  \"2.3.0\")'); then gem install bundler --version 1.17.3; fi"
   - gem --version
 before_script:
   - echo `whereis zip`

rubyzip.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |s|
   s.test_files            = Dir.glob('test/**/*')
   s.require_paths         = ['lib']
   s.license               = 'BSD 2-Clause'
-  s.required_ruby_version = '>= 1.9.2'
+  s.required_ruby_version = '>= 2.3'
   s.add_development_dependency 'rake', '~> 10.3'
   s.add_development_dependency 'pry', '~> 0.10'
   s.add_development_dependency 'minitest', '~> 5.4'