Update dotenv requirement from ~> 2.7 to ~> 3.1

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/main/Changelog.md)
- [Commits](bkeepers/dotenv@v2.8.1...v3.1.4)

---
updated-dependencies:
- dependency-name: dotenv
  dependency-type: direct:development
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    opsgenie-schedule (0.1.4)
+    opsgenie-schedule (0.1.6)
       httparty (~> 0.17)
 
 GEM
@@ -16,7 +16,7 @@ GEM
       bigdecimal
       rexml
     diff-lcs (1.5.1)
-    dotenv (2.8.1)
+    dotenv (3.1.4)
     hashdiff (1.1.1)
     httparty (0.21.0)
       mini_mime (>= 1.0.0)
@@ -79,7 +79,7 @@ PLATFORMS
 
 DEPENDENCIES
   bundler (~> 2.0)
-  dotenv (~> 2.7)
+  dotenv (~> 3.1)
   opsgenie-schedule!
   pry (~> 0.14.0)
   rake (~> 12.3.3)

opsgenie-schedule.gemspec

@@ -26,7 +26,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "rspec", "~> 3.0"
   spec.add_development_dependency "standard", "~> 0.1"
   spec.add_development_dependency "webmock", "~> 3.5"
-  spec.add_development_dependency "dotenv", "~> 2.7"
+  spec.add_development_dependency "dotenv", "~> 3.1"
 
   spec.add_dependency "httparty", "~> 0.17"
 end