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