travis: update to 1.6 and remove GO15VENDOREXPERIMENT as we don't vendor anymore

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 2430b4a..a9c492d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,7 +2,7 @@
 
 go:
   - 1.5.3
-  - 1.6rc2
+  - 1.6
   - tip
 
 matrix:
@@ -23,10 +23,6 @@
 before_install:
   - ssh-keyscan -t rsa -T 30 -H bitbucket.org | tee -a $HOME/.ssh/known_hosts
 
-env:
-  global:
-    - GO15VENDOREXPERIMENT="1"
-
 install:
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install bazaar; fi