Service events scheduling

[tzununbekov]

Changes:

• serverless.yaml manifest supports events schedule:

functions:
  go-lambda:
    source: go-lambda/main.go
    runtime: https://raw.githubusercontent.com/triggermesh/knative-lambda-runtime/master/go-1.x/runtime.yaml
    environment:
      foo: bar
    schedule:
    - cron: "*/5 * * * *"
      jsondata: '{"name":"triggermesh"}'
    - cron: "0 0 * * 6"
      jsondata: '{"weeklyBackup":true}'
      description: "some scheduled action"

• as a part of required changes knative dependencies were update, eventing legacy client removed ✨
• integration tests updates

closes #194

[tzununbekov]

I used v0.15.0 knative version since it brought less conflicts to this PR, which scope initially was not about deps update. I'll update dependencies to the latest versions as a part of a separate PR

[antoineco]

Really good stuff! A few small comments, mostly about code quality.

[antoineco]