bump cli to master

Bump cli to v0.15.3

update cli dep

Add initial docs for CAS.

Fix unit test.

Update go.step.sm to v0.5.0

Solves the problem of enforcing the signature algorithm. This
causes issues if the intermediate key is not an ECDSA key.

revert to skip_cleanup in travis

revert to skip_cleanup in travis

Update go.step.sm/crypto to v0.2.0

Fixes smallstep#302

go mod tidy and golang 1.15.0 cleanup ...

- cs.NegotiatedProtocolIsMutual has been deprecated but we still build
in travis with 1.14 so for now we'll ignore this linting error
- string(int) was resolving to string of a single rune rather than
string of digits -> use fmt.Sprint