This PR adds support for renovating the `node_js` versions in `.travis.yml` configuration files. Important notes:
- Functionality is disabled by default and hence opt-in via configuration
- Added a new manager type `node` because it is anticipated to support more than just Travis in future, with mostly unified logic
- Added the config option "policy" with supported values: lts, active, current, lts_latest and lts_active
- Policy is actually an array, to allow additive combining, e.g. `["lts_latest", "current"]`
- Actual node versions are *hardcoded*. There is no perfect metadata source for this and they change infrequently enough that it is definitely not a problem for now (next change will be in April 2018)
- If node versions need updating, they are listed from newest to oldest
- Replacing function attempts to detect the indention (spacing) in file and use that

To enable, configure `node.enabled=true` and optionally `node.policy=["<policy>"]` if you want something other than `lts`.

Closes #1208

Use `upath` for windows-friendly path joins.

Closes #1203

* refactor: don’t store yarn.lock content in packageFile

* chore: don’t log full lockfile

* refactor: don’t save package-lock.json

* update tests

Speeds up monorepos substantially

Remove original getFile, getFileContent, and getFileJson and replace with getFile (which used to be getFileContent.. shortened now for convenience). Also remove unused getSubdirectories functions.

Repository logic code has been rearranged for better logic and file separation.

Closes #654