Add check to ensure site is using WordPress 4.8

[aaronjorbin]

Fixes #1290

[westonruter]

There's a doubled semicolon here.

[aaronjorbin]

Thanks.

[nylen]

deactivate_plugins( array( 'gutenberg/gutenberg.php' ) );

I would not have thought to deactivate the plugin here. Is this common practice, or would it be better to keep this notice around?

[aaronjorbin]

@nylen I think it's best to deactivate the plugin so that it's very clear that Gutenberg is not in use and then after WordPress has been updated, the user can activate Gutenberg and see it in all its might.

[nylen]

A few fixes in the latest commits, including moving the PHP version check to a separate file, with the goal of keeping the gutenberg.php entry point extremely simple and short.

Moving the version check to its own function also fixes the build, because the plugin was loaded via a function during PHPUnit runs, which meant that the $wp_version global was not available at the top level of the code.

[nylen]

Thanks for working on this. It's working well (tested by temporarily changing $wp_version in my dev install); let's merge it.