CI: use conda-forge on 3.6 build
cc @zym1010
xref https://github.com/pandas-dev/pandas/issues/15662 - build failures on scipy 0.19.0

cc @kernc 
seems some failures on scipy 0.19.0 with object arrays in sparse (I guess warning < 0.19.0) # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15668?src=pr&el=h1) Report
> Merging [#15668](https://codecov.io/gh/pandas-dev/pandas/pull/15668?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7c5ebd50bbf5b659c6b40205bea5b42dbc892699?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.


```diff
@@            Coverage Diff             @@
##           master   #15668      +/-   ##
==========================================
- Coverage   91.03%   91.01%   -0.03%     
==========================================
  Files         143      143              
  Lines       49382    49382              
==========================================
- Hits        44957    44946      -11     
- Misses       4425     4436      +11
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15668?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/compare/7c5ebd50bbf5b659c6b40205bea5b42dbc892699...57e7edc69b16caaf224b0e38469c7e8daac44218?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :x: |
| [pandas/tools/merge.py](https://codecov.io/gh/pandas-dev/pandas/compare/7c5ebd50bbf5b659c6b40205bea5b42dbc892699...57e7edc69b16caaf224b0e38469c7e8daac44218?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL21lcmdlLnB5) | `93.61% <0%> (-0.34%)` | :x: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/7c5ebd50bbf5b659c6b40205bea5b42dbc892699...57e7edc69b16caaf224b0e38469c7e8daac44218?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.87% <0%> (-0.1%)` | :x: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15668?src=pr&el=footer). Last update [7c5ebd5...6a0d500](https://codecov.io/gh/pandas-dev/pandas/compare/7c5ebd50bbf5b659c6b40205bea5b42dbc892699...6a0d5000727b9e29bd3d6e2c45d79ef36911f59d?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).