(MAINT) Remove version constraint for rake

Prior to this commit the version of rake installed with this project was
vulnerable to an OS command injection attach.

The CVE ID for this is: CVE-2020-8130

This commit fixes the above by removing the version constraint and
ensuring that the latest version of rake is always pulled.

Author: chelnak

Gemfile

@@ -3,7 +3,7 @@ source 'https://rubygems.org'
 gemspec
 
 group :test do
-  gem 'rake', '~> 10.0'
+  gem 'rake'
   gem 'rspec-its', '~> 1.0'
   gem 'rspec-collection_matchers', '~> 1.0'