Appraise pull requests when a user signs a CLA.

Introduce Curry::PullRequestAppraiser to handle the use case of when a
user signs an ICLA or CCLA. The appraiser worker is called when a CLA is
created.
Rename UnknownCommitter to CommitAuthor

We now track whether or not a commit author has signed a cla using a
flag. This makes it possible to find other commit authors who haven't
signed a CLA on a PR opened by a user who has just signed a CLA.

In the future, we may periodically remove commit authors who have signed
a CLA.
Verify commit authors when they sign a CLA.

* Signing a CLA runs a job to verify commit authors
* Implement logic for unknown commit authors of a pull request
* Do not import known commit authors more than once
Add redis and GitHub access token to Travis config

Note: the GitHub access token is encrypted. B)