Update to go 1.15 (#7798)

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

.circleci/config.yml

@@ -11,7 +11,7 @@ executors:
   # should also be updated.
   golang:
     docker:
-    - image: circleci/golang:1.14-node
+    - image: circleci/golang:1.15-node
 
   fuzzit:
     docker:

.promu.yml

@@ -1,7 +1,7 @@
 go:
     # Whenever the Go version is updated here,
     # .circle/config.yml should also be updated.
-    version: 1.14
+    version: 1.15
 repository:
     path: github.com/prometheus/prometheus
 build:
@@ -30,7 +30,6 @@ crossbuild:
         - linux/amd64
         - linux/386
         - darwin/amd64
-        - darwin/386
         - windows/amd64
         - windows/386
         - freebsd/amd64