Expand supported Ruby versions to include 2.7

Author: jgarber623

.travis.yml

@@ -4,6 +4,7 @@ rvm:
   - 2.4
   - 2.5
   - 2.6
+  - 2.7
 cache: bundler
 before_script:
   - curl -L https://codeclimate.com/downloads/test-reporter/test-reporter-latest-linux-amd64 > ./cc-test-reporter

link-header-parser.gemspec

@@ -4,7 +4,7 @@ $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 require 'link_header_parser/version'
 
 Gem::Specification.new do |spec|
-  spec.required_ruby_version = ['>= 2.4', '< 2.7']
+  spec.required_ruby_version = ['>= 2.4', '< 2.8']
 
   spec.name          = 'link-header-parser'
   spec.version       = LinkHeaderParser::VERSION