fix: Add npm install

[BronsonQuick]

Summary

The Travis script is missing npm install which means Travis will show:

vuepress build docs
sh: 1: vuepress: not found

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[kefranabg]

@BronsonQuick nice catch!

[flozero]

As we saw with @kefranabg it's just for docs no need of package-lock my bad

[adrianjost]

I think npm install is the default install script and you don't need to specify it.
https://docs.travis-ci.com/user/languages/javascript-with-nodejs/#what-this-guide-covers

https://dev.to/hankchanocd/comment/5i5b

Nevertheless the install command should be placed in the install section of the Travis config.

install:
  - npm install
script:
  - npm run docs:build

Also keep in mind that npm ci might be a better solution: https://docs.travis-ci.com/user/languages/javascript-with-nodejs/#npm-ci-support

[vue-bot]

Hey @BronsonQuick, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚

[flozero]

thank you all for the work. @adrianjost i agree with you we should iterate on your comment. Can you create a pr to update it ?