Travis CI: Remove PHP7 from allowed failures list and test PHP nightly builds

[netweb]

PHP 7 has now been officially released:

​https://twitter.com/official_php/status/672511111896432640

We should update our Travis CI to not allow failures for PHP 7 and test PHP 7 nightly builds to allow to fail.

See also ​#WP33410 and ​https://make.wordpress.org/core/2015/09/10/wordpress-and-php7/

[netweb]

See also ​#bbPress2895 and ​changeset:5958

[netweb]

There be explosions with PHP 7 and WordPress versions < 4.3, 6758.1.diff​ excludes WordPress versions < 4.3 from the Travis CI test matrix.

[DJPaul]

What kind of explosions? Issues with PHP7 in WordPress core? A bit more info would have been useful. ;)

[DJPaul]

I was able to update my dev env to PHP7 and did some light testing, so happy to add this in (now that at the least I'd be able to look at any failures).

[djpaul]

In 10400:

Travis-CI: run tests on PHP7 and remove from allowed failures.

Add exceptions for particular versions of WordPress which pre-date that project's PHP7 compatibility fixes.

Props netweb
Fixes #6758

[netweb]

Replying to DJPaul:

What kind of explosions? Issues with PHP7 in WordPress core? A bit more info would have been useful. ;)

Thanks for getting this merged, an sorry, my bad, I didn't look much further than the PHP4 style constructors errors as they were the 1st things I noticed when I add this for same change for bbPress, and hence none of the PHP 7 work was back ported to WP versions pre 4.3 so once I had excluded those from the Travis build matrix all was resolved.

See also: ​https://make.wordpress.org/core/2015/07/02/deprecating-php4-style-constructors-in-wordpress-4-3/

And PHP 7 job errors here for bbPress: ​https://travis-ci.org/ntwb/bbPress/builds/94749040