Fix source location for pod spec

@nicklockwood, I realized that I needed to fix the source URL in the `GZIP.podspec`. I hope you don't mind merging this **and then** following a couple instructions to get this added as a CocoaPod.

First of all, your 1.0 tag needs to be deleted and then recreated with this latest commit (which would be the merge commit of this pull-request), otherwise the podspec will not work. Here's how to do it if you've never done it before.

```
# cd into the GZIP directory and then run these commands
git tag -d 1.0
git push origin :refs/tags/1.0
```

After that, you'll need to go to [releases](https://github.com/nicklockwood/GZIP/releases) and create a new one with the tag name "1.0". Just let me know when you do it, I can add the `podspec` to the master CocoaPods repository.

If this doesn't make sense or you need help, just let me know.