Include completion files for npm

Currently running `gulp --completion=[zsh|bash]` will not work because the completion files are being included in npm. By including the completion directory in the files array in package.json they will be installed by npm.

```sh
Specified gulp shell auto-completion rules for '[zsh|bash]' not found
```

---

**CHANGES:**
- Adds completion dir to package.json files

---

Let me know if this PR needs anything else