Update ristretto to commit f66de99 (#1391)

The update brings following changes from ristretto into badger

```
f66de99 Improve handling of updated items (dgraph-io/ristretto#168)
aec7994 Fix flaky TestCacheDel test (dgraph-io/ristretto#169)
a093fe6 Add benchmark plots to the project repo (dgraph-io/ristretto#166)
49dc42c Add Anurag as codeowner (dgraph-io/ristretto#158)
7a3f2d3 z: use MemHashString and xxhash.Sum64String (dgraph-io/ristretto#153)
9c31bb2 Check conflict key before updating expiration map. (dgraph-io/ristretto#154)
62cb731 Fix race condition in Cache.Clear (dgraph-io/ristretto#133)
9af1934 Docs and whitespace changes for readability. (dgraph-io/ristretto#143)
dbc185e Add changelog. (dgraph-io/ristretto#142)
ff325ad Remove key from policy after TTL eviction (dgraph-io/ristretto#130)
2dd5ff5 Use the require library in all the tests. (dgraph-io/ristretto#128)
7c48141 Use require in all tests in cache_test.go (dgraph-io/ristretto#127)
51e97ad Sets with TTL (dgraph-io/ristretto#122)
d3e7c37 Add benchmarks for math.rand and fastrand (dgraph-io/ristretto#118)
593823e Integrate fixes from PR dgraph-io/ristretto#91. (dgraph-io/ristretto#126)
29b4dd7 Fix comments. (dgraph-io/ristretto#123)
ddf345c Removed workflows directory. (dgraph-io/ristretto#124)
eb104d0 Add martinmr to CODEOWNERS file. (dgraph-io/ristretto#125)
```

(cherry picked from commit 09dfa66)

go.mod

@@ -5,7 +5,7 @@ go 1.12
 require (
 	github.com/DataDog/zstd v1.4.1
 	github.com/cespare/xxhash v1.1.0
-	github.com/dgraph-io/ristretto v0.0.2-0.20200115201040-8f368f2f2ab3
+	github.com/dgraph-io/ristretto v0.0.3-0.20200630154024-f66de99634de
 	github.com/dgryski/go-farm v0.0.0-20190423205320-6a90982ecee2
 	github.com/dustin/go-humanize v1.0.0
 	github.com/golang/protobuf v1.3.1

go.sum

@@ -13,8 +13,8 @@ github.com/cpuguy83/go-md2man v1.0.10/go.mod h1:SmD6nW6nTyfqj6ABTjUi3V3JVMnlJmwc
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/dgraph-io/ristretto v0.0.2-0.20200115201040-8f368f2f2ab3 h1:MQLRM35Pp0yAyBYksjbj1nZI/w6eyRY/mWoM1sFf4kU=
-github.com/dgraph-io/ristretto v0.0.2-0.20200115201040-8f368f2f2ab3/go.mod h1:KPxhHT9ZxKefz+PCeOGsrHpl1qZ7i70dGTu2u+Ahh6E=
+github.com/dgraph-io/ristretto v0.0.3-0.20200630154024-f66de99634de h1:t0UHb5vdojIDUqktM6+xJAfScFBsVpXZmqC9dsgJmeA=
+github.com/dgraph-io/ristretto v0.0.3-0.20200630154024-f66de99634de/go.mod h1:KPxhHT9ZxKefz+PCeOGsrHpl1qZ7i70dGTu2u+Ahh6E=
 github.com/dgryski/go-farm v0.0.0-20190423205320-6a90982ecee2 h1:tdlZCpZ/P9DhczCTSixgIKmwPv6+wP5DGjqLYw5SUiA=
 github.com/dgryski/go-farm v0.0.0-20190423205320-6a90982ecee2/go.mod h1:SqUrOPUnsFjfmXRMNPybcSiG0BgUW2AuFH8PAnS2iTw=
 github.com/dustin/go-humanize v1.0.0 h1:VSnTsYCnlFHaM2/igO1h6X3HA71jcobQuxemgkq4zYo=