Update build to use Go 1.15 (#236)

Co-authored-by: Tyler Yahn <[email protected]>

Author: ilmanzo

.circleci/config.yml

@@ -2,7 +2,7 @@ version: 2.1
 jobs:
   build:
     docker:
-      - image: circleci/golang:1.13
+      - image: circleci/golang:1.15
 
     environment:
       TEST_RESULTS: /tmp/test-results # path to where test results will be saved

exporters/metric/cortex/cortex_test.go

@@ -70,7 +70,7 @@ var mockTime int64 = time.Time{}.Unix()
 
 func TestExportKindFor(t *testing.T) {
 	exporter := Exporter{}
-	got := exporter.ExportKindFor(nil, aggregation.Kind(0))
+	got := exporter.ExportKindFor(nil, aggregation.Kind(rune(0)))
 	want := metric.CumulativeExporter
 
 	if got != want {