Use go 1.7

Update travis/circle CI builds to use Go 1.7.

.travis.yml

@@ -2,7 +2,7 @@ sudo: false
 
 language: go
 go:
-- 1.5
+- 1.7.x
 - tip
 
 script:

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-base
+    DOCKER_TEST_IMAGE_NAME: quay.io/prometheus/golang-builder:1.7-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'