Generate swift 4.2 compatible build of 2.5 Couchbase Lite iOS

We will need to release a  Xcode 10.1 / swift 4.2 compatible version of Couchbase Lite in addition to our currently supported Xcode 10.2 generated version

Background: 

2.5 release of Couchbase Lite supports swift 5.0.1 built using Xcode 10.2. This cannot be used to build applications with Xcode 10.1 / swift 4.2. 

The major issue because Xcode 10.2 requires an macOS upgrade which not all customers are ready to do.

Since swift 5 is relatively new, not all of our customers have ready to move to swift 5. Which means that many of them are still on pre Xcode 10.2. 

In short, we have users who are on swift 4 and swift 5 and the two are not compatible and swift4 users cannot upgrade to 2.5 for this reason

Current Expectation : We will not support multiple version downloads via carthage and cocoapods because these systems don't have an easy way for enabling that. This will be downloadable directly via our docs page via a "Looking for swift 4.2 compatible version? Download here) (since our website will only support one version download).