Support nil *Cache values in Clear and Close

[damz]

This change is 

[claassistantio]

All committers have signed the CLA.

[damz]

Unfortunately CI is broken in this repository, so I had to fix a test that seems to have been broken since at least f823dc4.

[jarifibrahim]

Looks good to me except one small comment. Thanks for the PR @damz 🥇

[jarifibrahim]

LGTM. Thanks, @damz. The Travis build is failing because we've set go 1.12 in go mod and we're trying to build ristretto in go 1.11. I think it should be okay to get rid of go 1.11 from the Travis build.

@martinmr Can you have a look, please?

[jarifibrahim]

Reviewable status: 0 of 3 files reviewed, all discussions resolved (waiting on @karlmcguire and @manishrjain)

[manishrjain]

Reviewed 1 of 1 files at r1, 2 of 2 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @karlmcguire)

[jarifibrahim]

Reviewed 1 of 1 files at r1, 2 of 2 files at r2, 1 of 1 files at r3.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @karlmcguire)

[martinmr]

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @karlmcguire)