Update to supported Ruby versions. (#13)

Set Ruby 2.6 as minimum version and removed support for Ruby 2.4 and 2.5

.rubocop.yml

@@ -1,5 +1,5 @@
 AllCops:
-  TargetRubyVersion: 2.4
+  TargetRubyVersion: 2.6
   NewCops: enable
   Exclude:
     - 'bin/**/*'

.travis.yml

@@ -5,11 +5,9 @@ language: ruby
 dist: focal
 os: linux
 rvm:
-  - 3.0.0
-  - 2.7.2
-  - 2.6.6
-  - 2.5.8
-  - 2.4.10
+  - 3.0.1
+  - 2.7.3
+  - 2.6.7
 cache: bundler
 before_script:
   - curl -L https://codeclimate.com/downloads/test-reporter/test-reporter-latest-linux-amd64 > ./cc-test-reporter

exiftool_vendored.gemspec

@@ -14,17 +14,7 @@ Gem::Specification.new do |spec|
   spec.homepage      = 'https://github.com/exiftool-rb/exiftool_vendored.rb'
   spec.license       = 'MIT'
 
-  spec.required_ruby_version = '>= 2.4'
-  spec.post_install_message = '
-    ***********************
-    * Ruby Support Notice *
-    ***********************
-
-    Starting March 31, 2021 releases of `exiftool_vendored` Gem will no longer support following
-    Ruby Versions due to their End Of Life (https://www.ruby-lang.org/en/downloads/branches/)
-    - Ruby 2.4 (EOL 2020-03-31)
-    - Ruby 2.5 (EOL 2021-03-31)
-  '
+  spec.required_ruby_version = '>= 2.6'
 
   spec.files         = `git ls-files -- lib bin`.split($/)