[backport] CI fixes (#6933)

* Relax shotgun test. (#6900)

It's non-deterministic algorithm, the test is flaky.

* Disable pylint error. (#6911)

* [CI] Skip external memory gtest on osx. (#6901)

* [CI] Fix custom metric test with empty dataset. (#6917)

* Reduce Travis environment setup time. (#6912)

* Remove unused r from travis.
* Don't update homebrew.
* Don't install indirect/unused dependencies like libgit2, wget, openssl.
* Move graphviz installation to conda.

* Relax shotgun test. (#6918)

* Relax test for decision stump in distributed environment. (#6919)

* Backport cupy fix.

tests/ci_build/conda_env/macos_cpu_test.yml

@@ -15,6 +15,7 @@ dependencies:
 - matplotlib
 - dask
 - distributed
+- graphviz
 - python-graphviz
 - hypothesis
 - astroid