Specify Go version to test with c++11

Default version of Go 1.11.1 on Travis is broken for c++11 testing.

gcc_libinit.c: In function ‘_cgo_try_pthread_create’:
gcc_libinit.c:110:3: error: implicit declaration of function ‘nanosleep’ [-Werror=implicit-function-declaration]
   nanosleep(&ts, nil);

.travis.yml

@@ -296,7 +296,7 @@ matrix:
       sudo: required
       dist: xenial
     - os: linux
-      env: SWIGLANG=go CPP11=1
+      env: SWIGLANG=go VER=1.6 CPP11=1
       sudo: required
       dist: trusty
     - os: linux