use git tag for versioning, reduce chance of inconsistency from  manual modification of .app.src file

Current versions of repo is from modification in vsn in app.src. This is error prone for release.
There is a plugin from erlware where you can use git tag to do that. This is NOT the git sha that we commonly encounter. 

(more information :https://github.com/erlware/rebar_vsn_plugin)
They have 2 styles: git version style (tag plus commit sha hash)
And sematic version style (just that tag)

Here are the proposed changes (please see file changes):
- include https://github.com/erlware/rebar_vsn_plugin.git in rebar.config
- in ap.src file vsn is git or "semver" rather than hand coded "1.0.0". The two styles are descriped rebar_vsn_plugin github link listed above.
The downside is that, you need to create the git  tag in proper format (see link) from where rebar_vsn_plugin will gets the value and stuff that in on compile.