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.
I believe what I have does that because it's in parentheses, so it's a subshell (http://www.linuxtopia.org/online_books/advanced_bash_scripting_guide/subshells.html). I actually started out with `pushd` and `popd`, but for some reason the `popd` was failing with an error so I went with the subshell instead.