Add warning message when Gemfiles use source with a block.
Merge pull request #3199 from danfinnie/stable-1-6-source-warning

Add warning message when Gemfiles use source with a block.
Document config for HTTPs source credentials
Fix a typo in the docs.
Reword credential docs for consistency and spelling
Version 1.6.7 with changelog
Merge tag 'v1.6.7'

Conflicts:
	CHANGELOG.md
	lib/bundler/dsl.rb
	lib/bundler/version.rb
Performance: lazily memoize expensive `prerelease?` values during resolve