Move test version back from 5.1.1 to 5.1 for PHP 5.4 tests

[schlessera]

To make the PHP 5.4 tests work, we have to limit them to the latest version before the 5.2 update (which bumps the PHP minimum version to PHP 5.6).

This is done by WP_VERSION=5.1, but that unfortunately does not include the latest stable patch version. This should be fixed through wp-cli/wp-cli-tests#51

In the mean-time, we bumped the version to WP_VERSION=5.1.1 to make the doctor tests pass that rely on checking for the latest stable version. This should be reverted back to WP_VERSION=5.1 once the above issue has been fixed.