Add tip to Travis CI build matrix

Run tests on the latest point release for Go 1.8, 1.9, and 1.10, so
1.10.3 instead of 1.10 for example. Also run tests on the master
branch.

.travis.yml

@@ -1,16 +1,18 @@
 language: go
+go_import_path: github.com/99designs/aws-vault
 
 install:
   - go get -u github.com/kardianos/govendor
   - govendor status
 
 go:
-  - "1.8"
-  - "1.9"
-  - "1.10"
+  - "1.8.x"
+  - "1.9.x"
+  - "1.10.x"
+  - "master"
 
 os:
   - linux
   - osx
-  
+
 osx_image: xcode7.3