Update TravisCI and README for Go 1.6.

Now that Go 1.6 has been released, update the required Go version in the
README to 1.5 and add Go 1.6 to the configurations tested by TravisCI.

Also, while here, update the Go 1.4 and 1.5 versions tested by TravisCI
to the latest point releases.

.travis.yml

@@ -1,7 +1,8 @@
 language: go
 go:
-  - 1.4.2
-  - 1.5.1
+  - 1.4.3
+  - 1.5.3
+  - 1.6
 sudo: false
 before_install:
   - gotools=golang.org/x/tools

README.md

@@ -35,7 +35,7 @@ active development.
 
 ## Requirements
 
-[Go](http://golang.org) 1.3 or newer.
+[Go](http://golang.org) 1.5 or newer.
 
 ## Installation