fix strip function for ssh urls

previous behavior
```js
strip('https://github.com/username/reponame')
// => username/reponame
strip('git@github.com:username/reponame')
// => git@github.com:/username/reponame
```

new behavior
```js
strip('https://github.com/username/reponame')
// => username/reponame
strip('git@github.com:username/reponame')
// => username/reponame
```