Basic example hangs in 0.4dev

Nobody has claimed this yet.

The basic example in the README hangs when using UMAP from the master or 0.4dev branch.

```python
import umap
from sklearn.datasets import load_digits

digits = load_digits()

embedding = umap.UMAP().fit_transform(digits.data)
```

If I do

```
pip install umap-learn
```

then the code works as expected (taking only 5-10 seconds). However, it hangs when using a version of UMAP built from the 0.4dev branch or master. It looks like it is doing *something* since I see the message

```
/home/gclenden/anaconda3/envs/umap_debug/lib/python3.7/site-packages/umap/spectral.py:243: UserWarning: Embedding a total of 2 separate connected components using meta-embedding (experimental)
n_components
```

I think the commit where this occurs is https://github.com/lmcinnes/umap/commit/eb6774056aa5fc8a858d466eaba4a1cb35c978ae. It works as expected on the commit prior to that one https://github.com/lmcinnes/umap/commit/c26bc8dac3e8fc71a4221986cf3cf0926b573766. However, I don't know enough about numba/tbb to explain why this might be happening.

Start with the README basic example and compare the changes between commits c26bc8dac3e8fc71a4221986cf3cf0926b573766 and eb6774056aa5fc8a858d466eaba4a1cb35c978ae. Inspect the spectral.py path shown in the warning and run the example against the affected branch. Done means the example completes rather than hanging, with behavior comparable to the released version.

Written by the indexing model from the issue text.