Skip running npm install when yarn requirement is not met

[ai]

I have a case of:

1. I have Node.js project with Yarn
2. I must test Node 10, 8, 6, 4, 0.12, 0.8.
3. Since most of the test tools don’t work on Node.js 0.12-0.8 I have custom install script which install dependencies only for Node 10-6.

Unfortunately, right now this case will not work with Travis since if Yarn doesn’t meet the requirement, Travis CI ignore custom install script and run npm install.

This PR should fix it and make this logic more predictable.

[BanzaiMan]

I don't think this is a change that we need to make.

I understand that your repo is having a little bit of difficulty, but I think in your case the best strategy is to skip install phases for older versions of node. See ai/nanoid#96

[BanzaiMan]

@ai Since you merged my PR, I'll assume that this PR is no longer necessary. :-)

[ai]

Since you merged my PR, I'll assume that this PR is no longer necessary. :-)

Thanks for ai/nanoid#96. But I do not think it could help.

npm install will be called even on install: skip, since install_yarn called it, not install.

My project’s tests are passed because I replace yarn to npm.

[ai]

PR was fixed (hope you will squash all commits)

[BanzaiMan]

Tested: https://staging.travis-ci.com/BanzaiMan/nanoid/builds/233110
Compare with: https://travis-ci.com/BanzaiMan/nanoid/builds/87954675