chore(ci): pin gh-pages to known working version

[vdice]

The most recent chart release initially failed, seemingly due to tschaub/gh-pages#354:

 $ k logs brigadecore-charts-publish-01ecx5q72k8j4wj3gcqkxb5834
/usr/local/bin/gh-pages -> /usr/local/lib/node_modules/gh-pages/bin/gh-pages.js
/usr/local/bin/gh-pages-clean -> /usr/local/lib/node_modules/gh-pages/bin/gh-pages-clean.js
+ gh-pages@3.1.0
added 52 packages from 14 contributors in 3.351s
The "path" argument must be of type string. Received undefined

Rerunning the job with this utility pinned to 3.0.0 worked:

 $ k logs brigadecore-charts-publish-01ecx77xnamn9yzr8sndjh1g35
/usr/local/bin/gh-pages -> /usr/local/lib/node_modules/gh-pages/bin/gh-pages.js
/usr/local/bin/gh-pages-clean -> /usr/local/lib/node_modules/gh-pages/bin/gh-pages-clean.js
+ gh-pages@3.0.0
added 41 packages from 14 contributors in 2.205s
Published

[krancour]

imho, this is better anyway because it's more deterministic.

[krancour]

LGTM