fix(version.txt): Move SDK version info from coreos-overlay to manifest

We need to continue to keep the SDK version separate from the current
build version because we need an existing built version to build new
releases from. cros_sdk and portage will now refer to this file for all
version information.

Change-Id: I55effa375244e289a553bff742f4c93e50f1835c
Merge pull request #7 from marineam/sdk-version

fix(version.txt): Move SDK version info from coreos-overlay to manifest