Prevent from loading if WP is installing

If someone is installing WP from scratch, but has already place WPSEO in the must-use (mu-plugins) directory, the plugin will throw errors as the database cannot be queried yet.

I believe this really is a core issue (and have reported it as such: [Trac 28668](https://core.trac.wordpress.org/ticket/28668), but in the mean time this should probably solve it.