ARROW-3466: [CI] Pin protobuf version to 3.6.0 [skip appveyor]

[pitrou]

Work around a crash with tensorflow and protobuf 3.6.1.

The change that triggered the crash seems to be the following:
protocolbuffers/protobuf#4878

[wesm]

Argh, our friend std::once. Can this be solved by using dynamic linking for Protobuf and bundling the .so?

[pitrou]

Hmm, what do you mean? Does Arrow use protobuf?

[pitrou]

ORC uses protobuf, though. And we link liborc statically. And it seems we let protobuf symbols leak out of libarrow.so...

[pitrou]

#2731 seems to fix the issue here.

[codecov-io]

Codecov Report

Merging #2730 into master will increase coverage by 0.95%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2730      +/-   ##
==========================================
+ Coverage   86.53%   87.49%   +0.95%     
==========================================
  Files         308      402      +94     
  Lines       43153    61454   +18301     
==========================================
+ Hits        37342    53768   +16426     
- Misses       5737     7612    +1875     
  Partials       74       74

Impacted Files	Coverage Δ
cpp/src/arrow/memory_pool.cc	67.27% <0%> (-0.82%)	⬇️
python/pyarrow/ipc.pxi	68.65% <0%> (ø)
python/pyarrow/tests/test_deprecations.py	100% <0%> (ø)
python/pyarrow/tests/test_plasma.py	95.89% <0%> (ø)
python/pyarrow/tests/pandas_examples.py	95.83% <0%> (ø)
cpp/src/arrow/python/io.cc	98.01% <0%> (ø)
python/pyarrow/hdfs.py	36.84% <0%> (ø)
python/pyarrow/io.pxi	60.59% <0%> (ø)
python/pyarrow/lib.pxd	0% <0%> (ø)
python/pyarrow/tests/test_cython.py	96.96% <0%> (ø)
... and 125 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ea96080...908f8cb. Read the comment docs.

[pitrou]

Closing in favour of #2731