Use HTTPS url for Pathogen git clone in README.

The current Pathogen URL in README is using the SSH clone URL, which gives me a good ole error:

```
Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
```

Using the HTTPS URL allows readers of the README to clone w/o having SSH keys setup or any of that stuff.