[License] Simplify the Texture license to be pure Apache 2 (removing ASDK-Licenses). (#1077)

* [License] Simplify the Texture license to be pure Apache 2 (removing ASDK-Licenses)

With permission of the Facebook Open Source team, we are simplifying the Texture
license so that clients can rely on the Apache 2 terms that most of Texture is
already covered by. This means that code originally forked from AsyncDisplayKit
will be re-licensed from "BSD 3-clause + PATENTS v2" to Apache 2 without a
PATENTS file.

After getting confirmation that the updates to these core files look good, we'll
propagate this new license header to all files (in this same PR) and get sign-off
from all parties before landing.

* [License] Update all Texture source files to be pure Apache 2.

* Changelog entry for Apache 2 license update.

* Revert "[License] Update all Texture source files to be pure Apache 2."

This reverts commit ffa0fbbba9.

* [License] Update all Texture source files to be pure Apache 2, maintaining copyrights.

* [License] Update CONTRIBUTING, README, Podspec & Dangerfile.

Texture.podspec

@@ -1,7 +1,7 @@
 Pod::Spec.new do |spec|
   spec.name         = 'Texture'
   spec.version      = '2.7'
-  spec.license      =  { :type => 'BSD and Apache 2',  }
+  spec.license      =  { :type => 'Apache 2',  }
   spec.homepage     = 'http://texturegroup.org'
   spec.authors      = { 'Huy Nguyen' => 'huy@pinterest.com', 'Garrett Moon' => 'garrett@excitedpixel.com', 'Scott Goodson' => 'scottgoodson@gmail.com', 'Michael Schneider' => 'schneider@pinterest.com', 'Adlai Holler' => 'adlai@pinterest.com' }
   spec.summary      = 'Smooth asynchronous user interfaces for iOS apps.'