Include completion files for npm

Currently running `gulp --completion=[zsh|bash]` will not work because the completion files are not 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
```

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**CHANGES:**
- Adds completion dir to package.json files

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Let me know if this PR needs anything else