Add more Go versions to CI; use base versions.

For Go, at least, the base versions seem to be regularly updated to point to
the tip patch release.
diff --git a/bitbucket-pipelines.yml b/bitbucket-pipelines.yml
index 3f46fda..d4b1a14 100644
--- a/bitbucket-pipelines.yml
+++ b/bitbucket-pipelines.yml
@@ -16,8 +16,14 @@
 pipelines:
   default:  # run on each push
     - step:
-        image: golang:1.11.8
+        image: golang:1.9
         <<: *Verify
     - step:
-        image: golang:1.12.3
+        image: golang:1.10
+        <<: *Verify
+    - step:
+        image: golang:1.11
+        <<: *Verify
+    - step:
+        image: golang:1.12
         <<: *Verify