Finalize publication and QA tooling

[weierophinney]

This patch incorporates each of:

• [Docs] Fixes #3 - Check All Headers In Documentation #8
• [Docs] Fixes #5 - Updates bullet list #9
• [Docs] Fixes #7 - Check For Blockquotes In Docs #10
• [Docs] Switch from Bookdown to MkDocs #11

and adds the following:

• All documentation was reviewed for content and formatting, and is now published to https://zendframework.github.io/zend-memory/
• Adds the encrypted GH token to the travis configuration
• Updates the QA tooling to:
  • remove the test bootstrap in favor of the composer autoloader
  • replace php-cs-fixer usage with phpcs
  • add composer scripts for testing, CS checks, etc.
  • update the travis configuration to reference these scripts and require PHP 7 builds to pass