fix: enforce node engine for node lts

[chadian]

Dependencies in oclif are being bumped and are breaking CLIs using non-supported node versions. To prevent this issues from becoming more widespread it's time to bump the engines property for oclif to the minimum LTS supported version.

For more information, see:

• Bug: Node 8 support is broken since version v1.3.0 errors#97
• https://oclif.io/blog/2019/10/31/oclif-node-updates
• https://oclif.io/docs/faqs#why-isnt-node-x-supported

[codecov]

Codecov Report

Merging #188 (285de2f) into master (25ea412) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #188   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files           2       2           
  Lines          12      12           
  Branches        3       3           
======================================
  Misses         12      12           

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[chadian]

There will be a broader plan to bump major versions and change engines across all the oclif packages.