Travis test failing due to missing dependencies and old nodejs version

[seriousme]

Hi,

I noticed you have a lot of greenkeeper pulls failing on Travis with a rather cryptic error
e.g. https://travis-ci.org/pouchdb/pouchdb-auth/jobs/208505418#L265

> pouchdb-plugin-helper "lint"
events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: spawn ENOENT
    at errnoException (child_process.js:1011:11)
    at Process.ChildProcess._handle.onexit (child_process.js:802:34)

The problem seems to be that Travis is instructed to use nodejs 0.10 whereas many dependencies advise to use nodejs >= 4.0

When using version 4.8.3 of Node.js. it becomes clear that eslint is not present on this particular build.

> pouchdb-auth@2.1.1 helper /home/hans/pouchdb-auth
> pouchdb-plugin-helper "lint"

events.js:141
      throw er; // Unhandled 'error' event
      ^

Error: spawn /home/hans/pouchdb-auth/node_modules/.bin/eslint ENOENT
    at exports._errnoException (util.js:907:11)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:189:32)
    at onErrorNT (internal/child_process.js:363:16)
    at nextTickCallbackWith2Args (node.js:511:9)
    at process._tickCallback (node.js:425:17)
    at Function.Module.runMain (module.js:443:11)
    at startup (node.js:140:18)
    at node.js:1043:3

In the other helper runs, its missing browserify, uglifyjs etc.

KR,
Hans