fix(config): move puppeteer from dependency to dev-dependency

[johnjbarton]

Fixes #3191
Thanks to @davidshepherd7 for suggesting this.

[johnjbarton]

@lusarz
This PR looks like it may cause
" Starting browser chrome (linux) on SauceLabs"
to fail. Could puppeteer cause this?

[lusarz]

@johnjbarton Do you have link to CI build where it fails ?

[johnjbarton]

I'm unsure what is going on
Here the pull request #3193 passes
https://travis-ci.org/karma-runner/karma/builds/444825046

Here the release fails
https://travis-ci.org/karma-runner/karma/builds/445228793

The full build history
https://travis-ci.org/karma-runner/karma/builds

[lusarz]

@johnjbarton Do you think it may be related with this part of code:

karma/test/client/karma.conf.js

Lines 65 to 69 in c097ecf

	if (TRAVIS_WITHOUT_BS) {
	browsers.push('Firefox')
	} else {
	browsers = Object.keys(launchers)
	}

and different env variables values for pr vs release build ?

[johnjbarton]

If we changed that conf.js to log the process env values we might get more info and anyway this kind of travis-only config should be logged since it can be so surprising.