Use as devDependency

I ran into an issue using xm as a devDependency. When running `npx xm dev` while it's intalled, I get the following when visiting the website:

```
₪ Serving / ...
obfuscated/node_modules/xm/utils.js:62
onMsg(child.stdout.toString());
^

TypeError: Cannot read property 'toString' of null
at spawn (obfuscated/node_modules/xm/utils.js:62:24)
at Server. (obfuscated/node_modules/xm/index.js:145:9)
at Server.emit (events.js:310:20)
at parserOnIncoming (_http_server.js:786:12)
at HTTPParser.parserOnHeadersComplete (_http_common.js:119:17)
```

It works fine of course if I don't have `xm` installed locally.

## Steps to reproduce:
1. On a fresh project `npm install --save-dev xm`
2. Run `npx xm dev`
3. Visit the address where the website is served.

Reproduce the issue with the listed fresh-project steps, then inspect utils.js:62 and the spawn call reached from index.js:145. Confirm the behavior when xm is installed as a devDependency, and consider the null stdout failure path. Done means npx xm dev serves the site without the reported TypeError.

Written by the indexing model from the issue text.