travis: only run go latest on OS X and include go tip, but allow failures

fixup

.travis.yml

@@ -3,12 +3,12 @@ sudo: false
 osx_image: xcode7.3
 os:
 - linux
-- osx
 go:
 - 1.5.4
 - 1.6.4
 - 1.7.4
 - 1.8
+- tip
 install:
 - git fetch --unshallow --tags
 - make vars
@@ -19,6 +19,12 @@ script:
 env:
   matrix:
     secure: gU8gCV9R8Kv/Gn0SmCP37edpfIbPoSvsub48GK7qxJdTU628H0KOMiZW/T0gtV5d67XJZ4eKnhJYlxwwxgSgfejO32Rh5GlYEKT/FuVoH0BD72dM1GDFLSrUiUYOdoHvf/BKIFA3dJFT4lk2ASy4Zh7SEoXHG6goBlqUpYx8hVA=
+matrix:
+  allow_failures:
+  - go: tip
+  include:
+  - os: osx
+    go: 1.8
 deploy:
   provider: script
   script: make travis_beta