Add a list of helpful extensions to README

Fixes #361
Provide a caveat link to aid in memory management

Fixes #427
Provide stronger language around vagrant-triggers in README

Fixes #378
Update CHANGELOG
Update phpMyAdmin to 4.2.13.1

4.3.1 is out, but I haven't done any testing with that. Let's
go with 4.2.13.1 and then push to 4.3.1 after we ship
Merge pull request #520 from Varying-Vagrant-Vagrants/phpmyadmin-update

[PR] Update phpMyAdmin to 4.2.13.1
Move from git:// to https:// for GitHub repositories

We can't yet move for develop.git.wordpress.org, as HTTPS does
not respond.
Merge pull request #521 from Varying-Vagrant-Vagrants/git-https

[PR] Move from git:// to https:// for GitHub repositories
s/instaleld/installed/
Merge pull request #519 from Varying-Vagrant-Vagrants/1.2-readme

[PR] Update README with various improvements for 1.2