*: bump Go version to 1.12 (#1329)

Signed-off-by: Simon Pasquier <spasquie@redhat.com>

.circleci/config.yml

@@ -6,7 +6,7 @@ executors:
   # also be updated.
   golang:
     docker:
-    - image: circleci/golang:1.11
+    - image: circleci/golang:1.12
 
 jobs:
   test:

.promu.yml

@@ -1,3 +1,7 @@
+go:
+    # Whenever the Go version is updated here, .travis.yml and
+    # .circle/config.yml should also be updated.
+    version: 1.12
 repository:
     path: github.com/prometheus/node_exporter
 build: