da.percentile and np.percentile mismatch

Test:
```python
import numpy as np
import dask.array as da
x = da.arange(12, chunks=2)
p = da.percentile(x, 50)
print("dask result", p.compute())
x = x.compute()
p = np.percentile(x, 50)
print("numpy result", p)
```

Result:
```
dask result [10.5]
numpy result 5.5
```

Versions:
```
Python 3.6.3
Dask 0.16.1 installed by pip3 install dask[complete] --user
numpy 1.14.0
```