Fix bug editing non-objectid _id in Ruby 1.8.7

Use a slice range instead of index, because old Ruby isn’t logical.

Ugh, OS X. Please ship a more modern version of Ruby. For the love.

Fixes #126
Bump version for 2.3.7
Merge branch 'hotfix/2.3.7' into develop

Conflicts:
	CHANGELOG.markdown
	genghis.php
	genghis.rb
	spec/requests/api_spec.rb