Allow merge to work consistently on windows

[lukeapage]

On windows, the default npm test doesn't work for me.. investigating I see no documentation that spawn sync works with running a command installed in a sub node_modules.

I'm not 100% sue, I just know that master doesn't work for me.

This issue was informative: nodejs/node#8077

[coreyfarrell]

@lukeapage Sorry to comment on a merged PR, I was AFK for almost 2 weeks.

In theory the path.join should not be needed as npm scripts are run with node_modules/.bin in the PATH, this is documented at https://docs.npmjs.com/cli/run-script.html. So it's not spawn sync that works with commands installed in node_modules, it's the fact that npm test is an alias to npm run test. Did you test this with shell: true but without using path.join?

[coreyfarrell]

Additional comment, we have travis-ci run tests in Windows and did not get any failure before this PR. I wonder why we don't get errors and if we can do something to our travis configuration to make it run tests in a mode which more closely resembles your local environment.