Bump minimum WP version to 3.9

[boonebgorges]

Previously: #6258

To update the stats reported at ​https://buddypress.trac.wordpress.org/ticket/6258#comment:9, here are the current usage numbers for each version of WordPress which BP currently has official support for, and which was released over one year ago:

3.8: 3.6% (2013-12-12)
3.9: 5.0% (2014-04-16)
4.0: 5.4% (2014-09-04)
4.1: 8.4% (2014-12-18)

I vote we act conservatively and drop official support for WP 3.8 in BP 2.5. Any objections? (By the looks of things, 2.6 may be able to bump WP requirements to 4.1.)

See also ​https://codex.buddypress.org/getting-started/wordpress-version-compatibility/

[imath]

+1. this ​https://buddypress.trac.wordpress.org/browser/trunk/src/bp-core/bp-core-attachments.php#L24 wouldn't be needed anymore :)

[DJPaul]

Sure, go for it.

Heads-up that my email stuff has a small reliance on 4.0-code, but we can talk about that when I finish that work.

[boonebgorges]

In 10436:

Bump minimum WP requirement to 3.9.

Fixes #6805.