Throw a notice when activating BP on a site that doesn't meet system requirements

[boonebgorges]

When BP requires a version of PHP higher than what WP requires, you could have people activating BP on a WP site that can't run BP. We should do the following:

• make sure our loader file is compatible with the minimum version of PHP supported by WordPress
• when BP is activated on a WP install that doesn't meet PHP requirements, throw an admin notice and don't load the rest of BP's files

[djpaul]

In 11206:

Update Composer PHP requirement to 5.3+, and shrinkwrap latest NPM packages.

See #7195

[djpaul]

In 11207:

Unit Tests: adieu, PHP 5.2.

BP 2.8 drops support for PHP 5.2. See #7195

[DJPaul]

7195.01.patch is really just a proof of concept of showing an admin notice if you're running PHP < 5.3 to start discussion.

I personally think we should intentionally use die to quit processing, so that WordPress falls back and never activates the plugin. See screenshot for a reminder of that behaviour: ​https://cloudup.com/cL4q1HTjVuo

[DJPaul]

I came across a Twitter discussion about this sort of thing, and found another plugin's approach to this: ​https://plugins.trac.wordpress.org/browser/cart-recovery/trunk/cart-recovery-for-wordpress.php#L40