Only support supported Go versions

As described on https://github.com/golang/go/wiki/Go-Release-Cycle

The maintainers do not test compatibility for libraries (e.g. in
golang.org/x/tools) on older versions.

.travis.yml

@@ -7,12 +7,9 @@ cache:
   - node_modules
 
 go:
-- 1.2.x
-- 1.3.x
-- 1.4.2
-- 1.5.x
 - 1.6.x
 - 1.7.x
+- 1.8.x
 - master
 
 matrix:
@@ -23,6 +20,8 @@ matrix:
     os: osx
   - go: 1.7.x
     os: osx
+  - go: 1.8.x
+    os: osx
 
 before_script:
 - go get github.com/urfave/gfmrun/... || true