Fix deprecation warning

Fix the following deprecation warning.

```
% bundle exec rake
[DEPRECATION] `last_comment` is deprecated.  Please use
`last_description` instead.
```

This warning was resolved by upgrade to Rubocop >= 0.38.0.

Author: koic

dotenv.gemspec

@@ -14,5 +14,5 @@ Gem::Specification.new "dotenv", Dotenv::VERSION do |gem|
 
   gem.add_development_dependency "rake"
   gem.add_development_dependency "rspec"
-  gem.add_development_dependency "rubocop", "~>0.37.2"
+  gem.add_development_dependency "rubocop", "~>0.40.0"
 end