chore(deps): update dependency mocha to v3 (#150)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

Gemfile

@@ -10,7 +10,7 @@ group :test do
   gem "rake"
   gem "minitest", "~> 5.3", "< 6.0"
   gem "minitest-stub-const"
-  gem "mocha", "~> 2.0"
+  gem "mocha", "~> 3.0"
 end
 
 group :development do