add go 1.12.x to the travis, circleci, and promu YAMLs (#5305)

Signed-off-by: tariqibrahim <tariq181290@gmail.com>

.circleci/config.yml

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

.promu.yml

@@ -1,7 +1,7 @@
 go:
     # Whenever the Go version is updated here, .travis.yml and
     # .circle/config.yml should also be updated.
-    version: 1.11
+    version: 1.12
 repository:
     path: github.com/prometheus/prometheus
 build:

.travis.yml

@@ -3,7 +3,7 @@ language: go
 # Whenever the Go version is updated here, .circleci/config.yml and .promu.yml
 # should also be updated.
 go:
-- 1.11.x
+- 1.12.x
 
 go_import_path: github.com/prometheus/prometheus