bump swift sdk version

Author: jamesnocentini

modules/ROOT/pages/swift.adoc

@@ -3,7 +3,7 @@
 :idseparator: -
 :snippet: {examplesdir}/swift/code-snippets/SampleCodeTest.swift
 :source-language: swift
-:version: 2.5.3
+:version: 2.6.0
 :blank-field: ____
 :url-issues-ios: https://github.com/couchbase/couchbase-lite-ios/issues
 :tabs: