Drop support for Go1.6

It will be handy to use testing.T.Run in the upcoming patches that add
more decode/encode tests. However since testing.T.Run was was added in
Go1.7 the code won't work with Go1.6 .

Since Go1.6 was released in the beginning of 2016 - more than 2.5 years
ago, and upstream Go policy is to support only current stable (currently
Go1.11) and two previous releases (currently Go1.10 and Go1.9), as of
today Go1.6 is both not supported and outdated. So let's drop explicit
support for it in the name of progress.

By above logic we could also drop support for 1.7 and 1.8, but since
there is no pressing particular need to do so I'm not dropping them for
now.

.travis.yml

 language: go
 
 go:
-  - 1.6
   - 1.7
   - 1.8
   - 1.9