refactor(build): Remove fxa-auth-server as a devDependency.

* fxa-auth-server is only needed for travis and slows down local `npm install`s. Install it on travis via the .travis.yml file.

fixes #971
chore(build): remove extra npm install
Merge pull request #972 from mozilla/issue-971-auth-server-dep

refactor(build): Remove fxa-auth-server as a devDependency.