Fix CI: ban sphinx-gallery 0.8.0

[sciunto]

Description

closes #4958

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[sciunto]

thank you @hmaarrfk for helping on this ;)

[sciunto]

Travis is still failing from time to time... :/

[sciunto]

As @hmaarrfk accepted and the PR is straightforward, I'm merging it.