Bump go version on travis to 1.8.3

 * Same as currently used in TB,
   https://gitweb.torproject.org/builders/tor-browser-build.git/tree/projects/go/config?id=8c4c05ddcfc30107b967b204d491523685460226

 * Also, necessary for the "context" import in acme/autocert

.travis.yml

@@ -12,7 +12,7 @@ addons:
             - gcc-5
 
 go:
-    - 1.6
+    - 1.8.3
 
 env:
     - TRAVIS_NODE_VERSION="4.2" CC="gcc-5" CXX="g++-5"