Set upperbound of matplotlib to avoid failure on Ubuntu

[HyukjinKwon]

This was caused from c7b6979. On Ubuntu 18, seems like there's a problem about packaging PIL (Pillow) somewhere, see also https://github.com/spotify/dh-virtualenv/issues/273.

From matplotlib 3.3.0, they started to use Pillow dependency (matplotlib/matplotlib@370e9a2#diff-60f61ab7a8d1910d86d9fda2261620314edcae5894d5aaa236b821c7256badd7)

This PR simply works around for now by setting uppberbound of matplotlib.

Resolves #1958

[codecov-io]

Codecov Report

Merging #1959 (24e6b9f) into master (138c7b8) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1959      +/-   ##
==========================================
- Coverage   94.64%   94.62%   -0.02%     
==========================================
  Files          49       49              
  Lines       10818    10834      +16     
==========================================
+ Hits        10239    10252      +13     
- Misses        579      582       +3     

Impacted Files	Coverage Δ
databricks/koalas/series.py	96.85% <0.00%> (-0.19%)	⬇️
databricks/koalas/namespace.py	84.19% <0.00%> (-0.04%)	⬇️
databricks/koalas/frame.py	96.72% <0.00%> (-0.03%)	⬇️
databricks/koalas/groupby.py	91.42% <0.00%> (ø)
databricks/koalas/missing/frame.py	100.00% <0.00%> (ø)
databricks/koalas/missing/indexes.py	100.00% <0.00%> (ø)
databricks/koalas/indexes.py	96.97% <0.00%> (+0.01%)	⬆️
databricks/koalas/internal.py	96.49% <0.00%> (+0.02%)	⬆️

---

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 138c7b8...24e6b9f. Read the comment docs.

[HyukjinKwon]

I think this is pretty straightforward. Let me merge this in.

[xinrong-meng]

Great, thank you! I will put this in the release note.

[itholic]

Nice!