Bump Go version 1.13.5 -> 1.15.5 and all dependencies

Author: bobrik

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.13.5-stretch
+FROM golang:1.15.5-buster
 
 # Doing mostly what CI is doing here
 RUN apt-get update && \

go.mod

@@ -1,18 +1,14 @@
 module github.com/cloudflare/ebpf_exporter
 
+go 1.15
+
 require (
-	github.com/alecthomas/units v0.0.0-20190924025748-f65c72e2690d // indirect
-	github.com/cespare/xxhash/v2 v2.1.1 // indirect
-	github.com/iovisor/gobpf v0.0.0-20200529092446-49b58e11a4b5
-	github.com/kr/pretty v0.1.0 // indirect
-	github.com/prometheus/client_golang v1.2.1
-	github.com/prometheus/client_model v0.0.0-20191202183732-d1d2010b5bee // indirect
-	github.com/prometheus/common v0.7.0
-	github.com/prometheus/procfs v0.0.8 // indirect
-	golang.org/x/sys v0.0.0-20191210023423-ac6580df4449
+	github.com/alecthomas/units v0.0.0-20201120081800-1786d5ef83d4 // indirect
+	github.com/iovisor/gobpf v0.0.0-20200614202714-e6b321d32103
+	github.com/prometheus/client_golang v1.8.0
+	github.com/prometheus/common v0.15.0
+	golang.org/x/sys v0.0.0-20201119102817-f84b799fce68
+	google.golang.org/protobuf v1.25.0 // indirect
 	gopkg.in/alecthomas/kingpin.v2 v2.2.6
-	gopkg.in/check.v1 v1.0.0-20190902080502-41f04d3bba15 // indirect
-	gopkg.in/yaml.v2 v2.2.7
+	gopkg.in/yaml.v2 v2.3.0
 )
-
-go 1.13