Check presence of each beam listed in .app

This allow to build riak, because one of riak deps (node_package) have stuff
in priv/ and valid node_package.app, but no one .beam files.
Merge pull request #42 from tsloughter/validate-app-file

Check presence of each beam listed in .app
keep application type of none for rel file
Merge pull request #43 from tsloughter/master

keep application type of none for rel file
back to travis for ci for now

Replacing README build status icon with travis until drone.io support PR builds.