travis: switch to using the .x version notation for the go minor versions

.travis.yml

@@ -4,10 +4,10 @@ osx_image: xcode7.3
 os:
 - linux
 go:
-- 1.6.4
-- 1.7.6
-- 1.8.5
-- 1.9.2
+- 1.6.x
+- 1.7.x
+- 1.8.x
+- 1.9.x
 - tip
 install:
 - git fetch --unshallow --tags
@@ -32,12 +32,12 @@ matrix:
   - go: tip
   include:
   - os: osx
-    go: 1.9.2
+    go: 1.9.x
     env: GOTAGS=""
 deploy:
   provider: script
   script: make travis_beta
   on:
     branch: master
-    go: 1.9.2
+    go: 1.9.x
     condition: "`uname` == 'Linux'"