Use latest minor version of Go 1.7

I am interested in the postgres_exporter releases built with any Go > 1.7.0 since that should fix the issue golang/go#16739 in Go.

.travis.yml

@@ -3,7 +3,7 @@ services:
 - docker
 language: go
 go:
-- '1.7'
+- '1.7.5'
 # Make sure we have p2 and the postgres client.
 before_install:
 - go get -v github.com/mattn/goveralls