Change version constraint in multi_json dependency

Upstream for multi_json suggests using pessimistic version constraints
with two digits for dependencies. In particular, upstream is now at
version 1.8.4, which breaks the given dependency, while all tests are
still successfull.

Gemfile

@@ -4,7 +4,7 @@ source 'https://rubygems.org'
 gemspec
 
 group :test do
-  gem 'multi_json', '~> 1.7.8'
+  gem 'multi_json', '~> 1.7'
   gem 'minitest'
   gem 'simplecov', :require => false
   gem 'simplecov-gem-adapter', :require => false