Increase minimum Node version to 6

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Node 4 loses LTS status on 2018-04-30.

[welcome]

Thanks for opening a pull request!

Here are some highlighted action items that will help get it across the finish line, from the
pull request guidelines:

• Follow the JavaScript coding style.
• Run npm run lint locally to catch formatting errors earlier.
• Document any user-facing changes in NEWS.md and other docs.
• Include tests when adding/changing behavior.

Development and triage is community-driven, so please be patient and we will get back to you as soon as we can.

[codecov]

Codecov Report

Merging #6 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #6   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           8      8           
  Lines         242    242           
=====================================
  Hits          242    242

Impacted Files	Coverage Δ
src/snapcraft.js	100% <ø> (ø)	⬆️
src/index.js	100% <100%> (ø)	⬆️
src/yaml.js	100% <100%> (ø)	⬆️

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