build: update to use go1.12rc1 for the build

.travis.yml

@@ -8,6 +8,7 @@ go:
 - 1.9.x
 - 1.10.x
 - 1.11.x
+- 1.12rc1
 - tip
 go_import_path: github.com/ncw/rclone
 before_install:
@@ -43,7 +44,7 @@ matrix:
   - go: tip
   include:
   - os: osx
-    go: 1.11.x
+    go: 1.12rc1
     env: GOTAGS=""
     cache:
       directories:
@@ -55,5 +56,5 @@ deploy:
   on:
     repo: ncw/rclone
     all_branches: true
-    go: 1.11.x
+    go: 1.12rc1
     condition: $TRAVIS_PULL_REQUEST == false