Test run seriously slowing down when renaming contracts folder

* Populus Version: 1.11.0
* OS: linux/ubuntu
* Python 3.5.2 (default, Sep 14 2017, 22:51:06)

### What was wrong?
consider folder structure:
```
contracts/
tests/
project.json
README.md
```
`tests` folder has 52 items to run, running them all with command `py.test -s --disable-pytest-warnings --populus-project=./ ./` takes ~85 seconds, maxing out load on one CPU core on my old laptop.

Now consider this folder structure:
```
ws_contracts/
tests/
project.json
README.md
```
Same files inside, `project.json` amended to reference the right folder.
Running same tests with the same command takes 4-5 times longer, also maxing out load on one CPU core on my old laptop. No apparent reason why.

Expected: renaming the contracts folder should not affect the test running time.

### Cute Animal Picture