ARROW-1456: [Python] Run s3fs unit tests in Travis CI

[rok]

This is to enable Python S3 tests in CI with minio as requested in ARROW-1456 and ARROW-4208.

[kszucs]

@ursabot build

[rok]

@ursabot build

[rok]

@kszucs, I'd like to add minio binary and python s3fs package to the docker image. This would be to enable S3 CI tests. Do I need to do something besides this PR?

[codecov-io]

Codecov Report

Merging #5200 into master will increase coverage by 4.32%.
The diff coverage is 94.44%.

@@             Coverage Diff             @@
##           master    #5200       +/-   ##
===========================================
+ Coverage   84.33%   88.65%    +4.32%     
===========================================
  Files         160      958      +798     
  Lines       10495   127472   +116977     
  Branches     1498     1498               
===========================================
+ Hits         8851   113016   +104165     
- Misses       1633    14091    +12458     
- Partials       11      365      +354

Impacted Files	Coverage Δ
python/pyarrow/tests/test_parquet.py	97.13% <100%> (ø)
python/pyarrow/tests/conftest.py	70.83% <66.66%> (ø)
python/pyarrow/ipc.pxi	80.87% <0%> (ø)
cpp/src/parquet/column_page.h	100% <0%> (ø)
cpp/src/plasma/test/external_store_tests.cc	100% <0%> (ø)
cpp/src/arrow/array/builder_decimal.cc	86.95% <0%> (ø)
cpp/src/plasma/client.cc	96.5% <0%> (ø)
cpp/src/arrow/compute/compute_test.cc	100% <0%> (ø)
cpp/src/arrow/python/io.cc	91.3% <0%> (ø)
python/pyarrow/hdfs.py	34.24% <0%> (ø)
... and 790 more

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[rok]

ARROW-6655 seems to do this and more. @kszucs shall I close this PR once ARROW-6655 is merged?

[kszucs]

@rok yes, we should close it in favor of ARROW-6655.

It introduces Minio for the filesystem bindings, and I've also updated the s3fs accordingly.