chore: pin node version used by Travis

[Airblader]

This pull request relates to or closes the following issues:

I have verified that…

• [ ] the commits in this pull request follow the conventionalcommits pattern
• [ ] tests have been updated or added
• [ ] documentation has been updated to reflect the changes made

[ghost]

DeepCode's analysis on #a69ac2 found:

• :x: 0 critical issues. :warning: 0 warnings and 0 minor issues. :heavy_check_mark: 0 issues were fixed.

:speech_balloon: This comment has been generated by the DeepCode bot, installed by the owner of the repository. The DeepCode bot protects your repository by detecting and commenting on security vulnerabilities or other critical issues.

---

:relaxed: If you want to provide feedback on our bot, here is how to contact us.

[codecov[bot]]

Codecov Report

Merging #147 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #147   +/-   ##
=======================================
  Coverage   84.77%   84.77%           
=======================================
  Files          25       25           
  Lines         742      742           
  Branches      117      117           
=======================================
  Hits          629      629           
  Misses         54       54           
  Partials       59       59

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update f560845...a69ac2f. Read the comment docs.