Only require the API/Express when using it

Fixes an issue with the Tessel, which cannot load Express.
Merge pull request #206 from hybridgroup/hotfix/tessel-express

Only require the API/Express when using it.
Bump version to 0.15.1
Merge branch 'master' into dev

* master:
  Bump version to 0.15.1
  Only require the API/Express when using it