Tag syntax for requiring a specific min version of WP

Looks like this `@require-wp-4.0`

Currently non-functional. Something's wrong with my bash

See #1357
Ends up like this, which breaks the test call:

```
+ BEHAT_TAGS='--tags='\''~@require-wp-4.0'\'''
+ vendor/bin/behat --format progress '--tags='\''~@require-wp-4.0'\'''
```

Because this current way works fine, I don't see any value in processing the echo'd value with regex, etc.