Make Figma API token optional in export script, fix CI issues

[shawnbot]

This is intended to fix #233 by working around the absence of the FIGMA_TOKEN environment variable on Travis. Here's how it works:

• If there is a FIGMA_TOKEN, we use that and fetch the icons from Figma as usual.
• Otherwise, we:
  1. Get the version field from lib/octicons_node/package.json
  2. Fetch the top-level package.json from GitHub at the versioned tag (https://raw.githubusercontent.com/primer/octicons/v${version}/package.json)
  3. If the figma.url field is the same as the one in the local top-level package.json, we can assume that the icons haven't changed in this build and fetch the build files from https://unpkg.com/octicons@${version}/build/
  4. Otherwise, we die. 💀

One issue that I ran into getting files from unpkg.com was a mysterious SSL/TLS error complaining about a self-signed cert. I think it's the same as this error, but none of the suggested solutions in that thread worked for me, so I worked around the Node's TLS by shelling out to curl -sL. ¯\_🤓_/¯

[shawnbot]

These lines are redundant.

[shawnbot]

This makes it easier to just run npx lerna run prepare from the top level.

[shawnbot]

I updated these to match the versions in octicons_gem/Gemfile.

[shawnbot]

TypeScript tests are run after the main test suite now.

[shawnbot]

FYI @jonrohan, our RUBYGEMS_USER environment variable was hidden in Travis, which is why previous builds had output that included strings like @github[secure]/octicons-react. I updated that variable to be displayed in the build logs so that it's easier to debug these package publish errors.

[shawnbot]

This used to throw an error if the package you specified didn't exist. If that's the case, just return 0.0.0.