Grab the latest here:
Android:
On GitHub –https://github.com/forcedotcom/SalesforceMobileSDK-Android/tree/v5.3.0
On npm – https://npmjs.org/package/forcedroid
On Bintray jCenter – https://bintray.com/forcedotcom/salesforcemobilesdk
iOS:
On GitHub –https://github.com/forcedotcom/SalesforceMobileSDK-iOS/tree/v5.3.0
On npm – https://npmjs.org/package/forceios
Release Notes:
Android –https://github.com/forcedotcom/SalesforceMobileSDK-Android/tree/v5.3.0#whats-new-in-53
iOS –https://github.com/forcedotcom/SalesforceMobileSDK-iOS/tree/v5.3.0#whats-new-in-53