Update dependency badges in README

[scottohara]

Now that
a) Gemnasium has been killed (acquired by GitLab)
b) Dependency CI has been killed (by Tidelift)
c) GitHub acquired Dependabot (security alerts)

...we ideally want to remove the Gemnasium/Dependency CI badges from the README; and replace with one or more badges from Github's own dependency checking/scanning services.

Ideally, we would want this badge (or badges) to show:
a) out of date dependencies (in both Gemfile and package.json)
b) security vulnerabilities

It doesn't seem (at this stage) that Github provides badges for these.

[scottohara]

Gemnasium & Dependency CI badges now removed.

Leaving this issue open for the other part (out of date dependencies & security vulnerabilities)

[scottohara]

Open issue for adding support for GitHub Dependabot badges: dependabot/dependabot-core#1912