Bump version number of WordPress in the README

Since `provision.sh` [links to latest.tar.gz](https://github.com/10up/varying-vagrant-vagrants/blob/master/provision/provision.sh#L295), it's now automatically pulling in 3.5.2 -- README is out of date.
Merge pull request #86 from kadamwhite/upgrade-wp-version-number

Bump version number of WordPress in the README