Increase minimum required WordPress version to 3.7

We're using core's unit testing framework and it doesn't run reliably below 3.7. It doesn't make sense to support old versions of WordPress indefinitely either.

.travis.yml

@@ -7,7 +7,7 @@ php:
 env:
     - WP_VERSION=latest
     - WP_VERSION=3.8
-    - WP_VERSION=3.3
+    - WP_VERSION=3.7
 
 before_script:
     - bash bin/install-wp-tests.sh wordpress_test root '' localhost $WP_VERSION 

readme.txt

@@ -2,7 +2,7 @@
 Contributors: batmoo, danielbachhuber, automattic
 Tags: authors, users, multiple authors, coauthors, multi-author, publishing
 Tested up to: 3.8.1
-Requires at least: 3.3
+Requires at least: 3.7
 Stable tag: 3.0.7
 
 Assign multiple bylines to posts, pages, and custom post types via a search-as-you-type input box
@@ -62,7 +62,7 @@ Bug fixes and minor enhancements
 * Guest author profile editor now supports a few different fields. Props [alpha1](https://github.com/alpha1).
 * Bug fix: Ensure `post_author` is set to one of the co-authors assigned to a post.
 * Packages a composer.json file for those using Composer.
-* Beginnings of unit test coverage for core features.
+* Beginnings of unit test coverage for core features. Increased minimum required WordPress version to 3.7 because WordPress.org unit testing framework doesn't work reliabilty below that.
 
 = 3.0.7 (Jan. 27, 2014) =
 * Better support for installing Co-Authors Plus as a symlinked directory. [Follow these instructions](http://kaspars.net/blog/wordpress/plugins-via-symlinks) to filter `plugins_url`.