Update ristretto to version 8f368f2 (#1195)

This commit pulls following changes from ristretto
```
git log c1f00be0418e...8f368f2 --oneline
```
```

8f368f2 (HEAD -> master) Fix DeepSource warnings
adb35f0 delete item immediately 
fce5e91 Support nil *Cache values in Clear and Close
4e224f9 Add .travis.yml 
8350826 Fix the way metrics are handled for deletions
99d1bbb (tag: v0.0.1) default to 128bit hashing for collision checks
1eea1b1 atomic Get-Delete operation for eviction
8d6a8a7 add cache test for identifying MaxCost overflows
ae09250 use the custom KeyToHash function if one is set
1dd5a4d #19 Fixes memory leak due to Oct 1st regression in processItems
```

(cherry picked from commit 82381ac)

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 v0.0.0-20191025175511-c1f00be0418e
+	github.com/dgraph-io/ristretto v0.0.2-0.20200115201040-8f368f2f2ab3
 	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 v0.0.0-20191025175511-c1f00be0418e h1:aeUNgwup7PnDOBAD1BOKAqzb/W/NksOj6r3dwKKuqfg=
-github.com/dgraph-io/ristretto v0.0.0-20191025175511-c1f00be0418e/go.mod h1:edzKIzGvqUCMzhTVWbiTSe75zD9Xxq0GtSBtFmaUTZs=
+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/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=

table/table.go

@@ -358,7 +358,10 @@ func (t *Table) readIndex() error {
 	y.Check(err)
 
 	t.estimatedSize = index.EstimatedSize
-	t.bf = z.JSONUnmarshal(index.BloomFilter)
+	if t.bf, err = z.JSONUnmarshal(index.BloomFilter); err != nil {
+		return y.Wrapf(err, "failed to unmarshal bloom filter for the table %d in Table.readIndex",
+			t.id)
+	}
 	t.blockIndex = index.Offsets
 	return nil
 }