Drop Go 1.3 from supported configurations

The code does not build on 1.3 because tls.Config does not contain the
GetCertificate field. This can be worked around using build constraints,
but it does not seem worth the trouble.

.travis.yml

@@ -3,7 +3,6 @@ sudo: false
 
 matrix:
   include:
-    - go: 1.3
     - go: 1.4
     - go: 1.5
     - go: 1.6