[CHEF-5356-gcm] Chef::EncryptedDataBagItem Version3 implementation using GCM
[CHEF-5356-gcm] Chef::EncryptedDataBagItem Version3 unit tests added
[CHEF-5356-gcm] Some small improvements to the EncryptedDataBag unit tests

Conflicts:
	spec/unit/encrypted_data_bag_item_spec.rb
[CHEF-5356-gcm] Chef::Config: Added a comment about EncryptedDataBag version 3
[CHEF-5356-gcm] Avoid testing GCM with ruby or OpenSSL versions
[CHEF-5356-gcm] docs updated to clarify ruby 2 requirement
[CHEF-5356-gcm] If the requirements to use Encryted Data Bags 3 are not met, give a meaningful error message
[CHEF-5356-gcm] Use OpenSSL::Cipher and OpenSSL::Digest instead of OpenSSL::Cipher::Cipher and OpenSSL::Digest::Digest
[CHEF-5356-gcm] Use rspec filters instead of conditionals and add some alternative tests using stubs
[CHEF-5356-gcm] moved some requires from decryptor.rb to assertions.rb
[CHEF-5356-gcm] spec/unit/encrypted_data_bag_item_spec.rb: small typo fix, "Ruby with_out_ AEAD support"
Merge pull request #1591 from onddo/CHEF-5356-gcm-2

[CHEF-5356-gcm(2)] Encrypted data bags should use different HMAC key and include the IV in the HMAC