Update dotenv requirement from ~> 1.0 to ~> 2.7

Updates the requirements on [dotenv](https://github.com/bkeepers/dotenv) to permit the latest version.
- [Release notes](https://github.com/bkeepers/dotenv/releases)
- [Changelog](https://github.com/bkeepers/dotenv/blob/master/Changelog.md)
- [Commits](bkeepers/dotenv@v1.0.0...v2.7.5)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

github-pages-health-check.gemspec

@@ -22,7 +22,7 @@ Gem::Specification.new do |s|
   s.add_dependency("public_suffix", ">= 2.0.2", "< 5.0")
   s.add_dependency("typhoeus", "~> 1.3")
 
-  s.add_development_dependency("dotenv", "~> 1.0")
+  s.add_development_dependency("dotenv", "~> 2.7")
   s.add_development_dependency("gem-release", "~> 0.7")
   s.add_development_dependency("pry", "~> 0.10")
   s.add_development_dependency("rspec", "~> 3.0")