Restore use of Ubuntu 14.04 for Linux CI builds

Cherry-pick atom#19617 onto 1.40-releases branch

Co-authored-by: Antonio Scandurra <as-cii@github.com>

Restore use of Ubuntu 14.04 for Linux CI builds

Cherry-pick atom#19617 onto 1.39-releases branch

Co-authored-by: Antonio Scandurra <as-cii@github.com>

Merge pull request atom#19681 from atom/github-up-dev

Upgrade GitHub package on dev

1.39.0

Merge pull request atom#19612 from atom/debounce-resize-event

Debounce the resize event handler

Merge pull request atom#19528 from atom/ns/fix-font-size-reset

Assign originalFontSize when the Workspace is initialized

Merge pull request atom#19528 from atom/ns/fix-font-size-reset

Assign originalFontSize when the Workspace is initialized

⬆️ tree-sitter to 0.15.4

⬆️ tree-sitter@0.15.4 to fix crashes on macOS < 10.12

⬆️ markdown-preview@0.160.2