Update Podspec.

Prepare 4.7.2. (Alamofire#2492)

* Update Gems.

* Version to 4.7.2.

* Update Changelog.

* Update Jazzy docs.

Update to 4.7.1. (Alamofire#2476)

* Update copyright to 2018.

* Version to 4.7.1, update CHANGELOG.

* More 2018 updates.

* Update Jazzy docs.

Update for 4.7 release.

4.6.0 Release. (Alamofire#2374)

* Prepare for 4.6.0 release.

* Match verbs to sections.

Updated the support versions of Swift, Xcode, and CocoaPods in the RE…

…ADME.

Added release notes to the CHANGELOG and bumped the version to 4.5.0.

Added release notes to the CHANGELOG and bumped the version to 4.4.0.

Cleaned up whitespace throughout the project…no functional changes.

Added release notes to the CHANGELOG and bumped the version to 4.2.0.