CI: Restore pyarrow for travis-3.6
Present in the last two builds on master

- https://travis-ci.org/pandas-dev/pandas/jobs/542915088
- https://travis-ci.org/pandas-dev/pandas/jobs/542931830

```
gw0 I
/home/travis/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/hypothesis/_settings.py:193: HypothesisDeprecationWarning: The timeout setting can safely be removed with no effect.
  verbosity=self.verbosity,
[gw0] node down: Not properly terminated
Replacing crashed worker gw0

```

Something is crashing our pytest-xdist worker. We used pytest-xdist 1.28.0 in the last green build https://travis-ci.org/pandas-dev/pandas/jobs/542868383, so it's likely not pytest-xdist itself.

Edit: Seems to be an issue with feather / pyarrow. We segfault when collecting tests: https://travis-ci.org/pandas-dev/pandas/jobs/542977519#L3052. https://github.com/pandas-dev/pandas/pull/26715 is skipping, use this issue to restore.

```
collecting 34877 items / 1 skipped / 34876 selected                            Fatal Python error: Segmentation fault
Current thread 0x00007fb9015dd740 (most recent call first):
  File "<frozen importlib._bootstrap>", line 219 in _call_with_frames_removed
  File "<frozen importlib._bootstrap_external>", line 922 in create_module
  File "<frozen importlib._bootstrap>", line 571 in module_from_spec
  File "<frozen importlib._bootstrap>", line 658 in _load_unlocked
  File "<frozen importlib._bootstrap>", line 955 in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 971 in _find_and_load
  File "/home/travis/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/pyarrow/__init__.py", line 47 in <module>
  File "<frozen importlib._bootstrap>", line 219 in _call_with_frames_removed
  File "<frozen importlib._bootstrap_external>", line 678 in exec_module
  File "<frozen importlib._bootstrap>", line 665 in _load_unlocked
  File "<frozen importlib._bootstrap>", line 955 in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 971 in _find_and_load
  File "/home/travis/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/_pytest/outcomes.py", line 168 in importorskip
  File "/home/travis/build/pandas-dev/pandas/pandas/tests/io/test_feather.py", line 13 in <module>
  File "/home/travis/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/_pytest/assertion/rewrite.py", line 304 in load_module
  File "<frozen importlib._bootstrap>", line 626 in _load_backward_compatible
  File "<frozen importlib._bootstrap>", line 656 in _load_unlocked
  File "<frozen importlib._bootstrap>", line 955 in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 971 in _find_and_load
  File "/home/travis/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/py/_path/local.py", line 701 in pyimport

```