Travis: Update to Go 1.10.3 for releases, and test on newer Go versions.

.travis.yml

@@ -1,6 +1,9 @@
 language: go
 go:
-  - "1.10"
+  - "1.10.3"
+  - "1.10.x"
+  - "1.x"
+  - "master"
 
 addons:
   apt:
@@ -26,4 +29,4 @@ env:
     # GITHUB_TOKEN for automatic releases
     - secure: "at1oJs7ib7glx3W+zk+OkT041LdknVXirIhN403CIihVUrlOhODY7yCTgvF4Rk0jYBJiT35Q2qxpgfWF2qGnsNsQmjG3ydDWQDCepDc/CgXfLyoiSTJK5vTK72dYWTVsBTycXbj1CbSy2X2ah/KWjc4RcgZ67ER7mDpRU5nFeow="
     # Set this to the Go version to use for releases (must appear in version list above).
-    - RELEASE_GO_VERSION="1.9"
+    - RELEASE_GO_VERSION="1.10.3"