Drop Rails 3.x support

[shirish-pampoorickal]

This PR introduces the following changes:

• Drop support for Ruby versions < 2.2.0
• Default to Ruby version 2.4.7
• Drop support for ActiveRecord versions < 4.2
• Add support for ActiveRecord versions 5.0, 5.1 & 5.2
• Upgrade Rspec & convert specs to use expect syntax
• Addressed following deprecations for version > Rails 5.1 while maintaining backward compatibility:
  • The behavior of 'attribute_changed?' inside of after callbacks will be changing in the next version of Rails. The new return value will reflect the behavior of calling the method after 'save' returned (e.g. the opposite of what it returns now). To maintain the current behavior, use 'saved_change_to_attribute?' instead.
  • The behavior of 'attribute_was' inside of after callbacks will be changing in the next version of Rails. The new return value will reflect the behavior of calling the method after 'save' returned (e.g. the opposite of what it returns now). To maintain the current behavior, use 'attribute_before_last_save' instead.

Bigmaven PR: https://github.com/mavenlink/mavenlink/pull/17371

TODO:

• Bump version

[plainprogrammer]

LGTM!