Output a SemVer compliant build version string

We currently use the `:git_describe` version for all chef-client 
projects. This build version format does not respect 
`Omnibus::Config.append_timestamp` which in turn causes two different 
builds to produce packages named identically. This can wreck havoc on 
when packages are published to backends like Omnitruck and Artifactory.
Merge pull request #192 from opscode/schisamo/use-semver-everywhere

Output a SemVer compliant build version string in all Client projects