Remove support for Go < 1.7

Package golang.org/x/tools/imports introduces a dependency to the `context` package, which
was added to the Go stdlib in Go v1.7.

See also:
golang/tools@165bdd6

Author: breml

.travis.yml

@@ -5,11 +5,6 @@ install: go get golang.org/x/tools/imports/...
 script: go test -v ./...
 
 go:
-    - 1.2.x
-    - 1.3.x
-    - 1.4.x
-    - 1.5.x
-    - 1.6.x
     - 1.7.x
     - 1.8.x
     - 1.9.x

README.md

@@ -18,7 +18,7 @@ Github user [@mna][6] created the package in April 2015, and [@breml][5] is the
 
 ### Breaking Changes since v1.0.0
 
-* Removed support for Go v1.1.x in []() due to an incompatibility of [golang.org/x/tools/imports](https://godoc.org/golang.org/x/tools/imports) with [go/ast](https://godoc.org/go/ast) of go v1.1.x.
+* Removed support for Go < v1.7 due to the requirement of the package `context` in [golang.org/x/tools/imports](https://godoc.org/golang.org/x/tools/imports), which was added to the Go stdlib in Go v1.7. This is in compliance with the [Go Release Policy](https://golang.org/doc/devel/release.html#policy) respectively the [Go Release Maintenance](https://github.com/golang/go/wiki/Go-Release-Cycle#release-maintenance), which states support for each major release until there are two newer major releases.
 
 ## Installation