use string property in travis.yml

using the string should fix that nvm can't find the node version
Merge pull request #2 from pxlpnk/patch-1

use string property in travis.yml
begin implementation of contentful view helper

- raise error if missing required config options
- set up contentful client
- configure content types from contentful api if `name` option is missing (default to underscored and pluralized output of content type name)
test view helper, stub contentful
set up coverall code coverage
a couple small edits, untested
test custom name and missing access token
promise fix in progress
repace sinon with mockery, :100: tests