travis: update golang versions

Now when golang 1.11 is out, 1.9 and older versions are no longer
supported. More to say, since the archive/tar is from go-1.11, it
uses some features from new Go versions (strings.Builder and sync.Map)
not supported by anything older than Go 1.10.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.travis.yml

@@ -1,11 +1,8 @@
 language: go
 go:
   - tip
-  - 1.x
-  - 1.8.x
-  - 1.7.x
-  - 1.6.x
-  - 1.5.x
+  - 1.11.x
+  - 1.10.x
 
 # let us have pretty, fast Docker-based Travis workers!
 sudo: false