Fix npm deprecation warnings

[feross]

Help diagnosing and fixing up these npm deprecation warnings would be most welcome! ✨

npm WARN deprecated opn@6.0.0: The package has been renamed to `open`
npm WARN deprecated material-ui@0.20.2: You can now upgrade to @material-ui/core
npm WARN deprecated os-homedir@2.0.0: This is not needed anymore. Use `require('os').homedir()` instead.
npm WARN deprecated cross-spawn-async@2.2.5: cross-spawn no longer requires a build toolchain, use it instead
npm WARN deprecated minimatch@2.0.10: Please update to minimatch 3.0.2 or higher to avoid a RegExp DoS issue
npm WARN deprecated core-js@1.2.7: core-js@<2.6.8 is no longer maintained. Please, upgrade to core-js@3 or at least to actual version of core-js@2.

[Borewit]

Progress (check if warning has been resolved on master branch):

• npm WARN deprecated opn@6.0.0: The package has been renamed to open: PR: #1667 rename dependency opn to open #1677
• npm WARN deprecated material-ui@0.20.2: You can now upgrade to @material-ui/core
• npm WARN deprecated os-homedir@2.0.0: This is not needed anymore. Use require('os').homedir() instead.
  • dependency of bubble: fixed in PR: Get rid of os-homedir dependency bublejs/buble#207, not released yet
• npm WARN deprecated cross-spawn-async@2.2.5: cross-spawn no longer requires a build toolchain, use it instead (already addressed?)
• npm WARN deprecated minimatch@2.0.10: Please update to minimatch 3.0.2 or higher to avoid a RegExp DoS issue
• npm WARN deprecated core-js@1.2.7: core-js@<2.6.8 is no longer maintained. Please, upgrade to core-js@3 or at least to actual version of core-js@2.

[alxhotel]

Last remaining warnings:

npm WARN deprecated material-ui@0.20.2: You can now upgrade to @material-ui/core
npm WARN deprecated core-js@2.6.12: core-js@<3.3 is no longer maintained and not recommended for usage due to the number of issues. Because of the V8 engine whims, feature detection in old core-js versions could cause a slowdown up to 100x even if nothing is polyfilled. Please, upgrade your dependencies to the actual version of core-js.