use peerDependencies instead of dependencies

Because the user might need a different version of dustjs linkedin or might have other
modules using his version of dust, this allows for more use cases to work properly
fix tests

peerDependencies are not installed when doing npm i in the project so we also need
to add dustjs-linkedin in the devDependencies to get tests running
Merge pull request #89 from qraynaud/master

use peerDependencies instead of dependencies for dustjs-linkedin module