Remove references to TravisCI

[nate-double-u]

Since the containternetworking/plugins project has moved away from Travis, we should remove references to it from the website as well.

• Plugins Overview page build status
• netlify.toml references travis-ci.org when checking dead links. Verify we don't need this any more.

  cni.dev/netlify.toml

  Lines 40 to 44 in 5373bbe

  	skipPatterns = [
  	"#containernetworking",
  	"travis-ci.org",
  	"coveralls.io",
  	]