AttributeError: module 'scipy.sparse' has no attribute 'csgraph'

Hello,

Thank you for the great contribution.

I can't seem to get it running. Any help is appreciated.

Here are my versions:
```
Requirement already satisfied: umap-learn in ./anaconda3/lib/python3.6/site-packages
Requirement already satisfied: numba>=0.34 in ./anaconda3/lib/python3.6/site-packages (from umap-learn)
Requirement already satisfied: scipy>=0.19 in ./anaconda3/lib/python3.6/site-packages (from umap-learn)
Requirement already satisfied: scikit-learn>=0.16 in ./anaconda3/lib/python3.6/site-packages (from umap-learn)
Requirement already satisfied: llvmlite in ./anaconda3/lib/python3.6/site-packages (from numba>=0.34->umap-learn)
Requirement already satisfied: numpy in ./anaconda3/lib/python3.6/site-packages (from numba>=0.34->umap-learn)
```

Running the example,
```
import umap
from sklearn.datasets import load_digits

digits = load_digits()

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

outputs:
```
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
in ()
4 digits = load_digits()
5
----> 6 embedding = umap.UMAP().fit_transform(digits.data)

~/anaconda3/lib/python3.6/site-packages/umap/umap_.py in fit_transform(self, X, y)

~/anaconda3/lib/python3.6/site-packages/umap/umap_.py in fit(self, X, y)

~/anaconda3/lib/python3.6/site-packages/umap/umap_.py in simplicial_set_embedding(graph, n_components, initial_alpha, a, b, gamma, negative_sample_rate, n_epochs, init, random_state, verbose)

~/anaconda3/lib/python3.6/site-packages/umap/umap_.py in spectral_layout(graph, dim, random_state)

AttributeError: module 'scipy.sparse' has no attribute 'csgraph'
```

But I can import csgraph witout problems from scipy,
`from scipy.sparse import csgraph
`

Reproduce the digits example with umap.UMAP().fit_transform(digits.data) and inspect the traceback through umap/umap_.py, especially spectral_layout. Compare the failing scipy.sparse.csgraph access with the separately working `from scipy.sparse import csgraph` import. Done means the example runs without the AttributeError.

Written by the indexing model from the issue text.