Authored by Bogdan Poplauschi

Bumped version to 4.2.1, updated CHANGELOG

## [4.2.0 - Pluginable coders, on Oct XX, 2017](https://github.com/rs/SDWebImage/releases/tag/4.2.0)
## [4.2.1 - 4.2 Patch, on Oct 31st, 2017](https://github.com/rs/SDWebImage/releases/tag/4.2.1)
See [all tickets marked for the 4.2.1 release](https://github.com/rs/SDWebImage/milestone/18)
#### Features
- Feature refactor built-in coders and support animated webp on macOS #2082 (reusable code into `SDWebImageCoderHelper`; `SDWebImageFrame` abstracts animated images frames)
#### Fixes
- Fixed EXIF orientation method will crash on iOS 7 because it’s an iOS 8 above API #2082
## [4.2.0 - Pluginable coders, on Oct 30th, 2017](https://github.com/rs/SDWebImage/releases/tag/4.2.0)
See [all tickets marked for the 4.2.0 release](https://github.com/rs/SDWebImage/milestone/16)
#### Features
... ...
Pod::Spec.new do |s|
s.name = 'SDWebImage'
s.version = '4.2.0'
s.version = '4.2.1'
s.osx.deployment_target = '10.8'
s.ios.deployment_target = '7.0'
... ...
... ... @@ -15,11 +15,11 @@
<key>CFBundlePackageType</key>
<string>FMWK</string>
<key>CFBundleShortVersionString</key>
<string>4.2.0</string>
<string>4.2.1</string>
<key>CFBundleSignature</key>
<string>????</string>
<key>CFBundleVersion</key>
<string>4.2.0</string>
<string>4.2.1</string>
<key>NSPrincipalClass</key>
<string></string>
</dict>
... ...