Add gulp ^3.0.0 as a peer dependency

Could't use `gulp-gh-pages` in `npm link` mode during development without adding `gulp` as a peer dependency.

Using `peerDependencies` as `gulp-gh-pages` is a plugin to `gulp`. Using version `^3.0.0` as `npm test` passed
for `3.0.0` as well as the latest `3.8.10` version. Leanient peer dependency versioning is encouraged.
http://blog.nodejs.org/2013/02/07/peer-dependencies/

Note: `gulp` could also be a dev dependency, if test are written for `index.js`. I don't think Travis uses
`peerDependencies`.