Merge pull request #118 from aalexand/aalexand-clarify-go-versions

Clarify that Go 1.7 or newer is required.

.travis.yml

 language: go
 go:
-  - 1.6.x
   - 1.7.x
   - 1.8.x

README.md

+[![Build Status](https://travis-ci.org/google/pprof.svg?branch=master)](https://travis-ci.org/google/pprof)
+
 # Introduction
 
 pprof is a tool for visualization and analysis of profiling data.
 
 Prerequisites:
 
-- Go development kit. Known to work with Go 1.5.
+- Go development kit. Requires Go 1.7 or newer.
   Follow [these instructions](http://golang.org/doc/code.html) to install the 
   go tool and set up GOPATH.