CBLT-268:  pass metadata to xcodebuild:

"xcodebuild CURRENT_PROJECT_VERSION=${BUILD_NUMBER} CBL_VERSION_STRING=${VERSION} CBL_SOURCE_REVISION=${REPO_SHA}"

Change-Id: Ic51005976fad6b10395c44556cb0d4682259b8e5
Reviewed-on: http://review.couchbase.org/36541
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>