Update promu config file following recent changes + use the golang-builder base image for tests for CircleCI

.promu.yml

@@ -1,3 +1,5 @@
+go:
+    cgo: true
 repository:
     path: github.com/prometheus/node_exporter
 build:

circle.yml

@@ -2,7 +2,7 @@ machine:
   environment:
     DOCKER_IMAGE_NAME: prom/node-exporter
     QUAY_IMAGE_NAME: quay.io/prometheus/node-exporter
-    DOCKER_TEST_IMAGE_NAME: quay.io/prometheus/golang-builder:1.6-main
+    DOCKER_TEST_IMAGE_NAME: quay.io/prometheus/golang-builder:1.6-base
     REPO_PATH: github.com/prometheus/node_exporter
   pre:
     - sudo curl -L -o /usr/bin/docker 'https://s3-external-1.amazonaws.com/circle-downloads/docker-1.9.1-circleci'