travis: drop older Go versions

Author: stapelberg

.travis.yml

@@ -1,8 +1,10 @@
 language: go
 go_import_path: pault.ag/go/debian
 go:
-  - 1.9.x
-  - 1.8.x
-  - 1.7.x
-  - 1.6.x
-  - 1.5.x
+  - "1.15.x"
+  - "1.14.x"
+  - "1.13.x"
+  - "1.12.x"
+  - "1.11.x"
+  - "1.10.x"
+  - "1.9.x"