deploy: Do pip install in $CURRENT_DIR

so "." in `requirements.txt` works

This allows one to have stuff in `requirements.txt` like:

```
-e .
```

Without this, I have to build a new package for my app, which makes it harder to update the app and takes away a lot of convenience of tsuru.
It would be good if we went back to the previous directory after running pip. You can use `pushd` / `popd` to do it.