Update Travis build (#89)

* Test with the latest releases.
* Allow tip to fail.

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 8e6ca75..5307ea8 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,13 +1,18 @@
 language: go
-script: "./test.sh"
 go:
-    - 1.5
-    - 1.6
-    - 1.7
-    - tip
+  - 1.5.4
+  - 1.6.3
+  - 1.7
+  - tip
+matrix:
+  allow_failures:
+    - go: tip
+  fast_finish: true
+script:
+  - ./test.sh
 before_install:
   - go get github.com/axw/gocov/gocov
   - go get github.com/mattn/goveralls
   - if ! go get code.google.com/p/go.tools/cmd/cover; then go get golang.org/x/tools/cmd/cover; fi
 after_success:
-    - $HOME/gopath/bin/goveralls -service=travis-ci
+  - $HOME/gopath/bin/goveralls -service=travis-ci