Add CircleCI build url to version.json. Fixes #186

scripts/echo_version_json.sh

@@ -1,4 +1,5 @@
-printf '{"commit":"%s","version":"%s","source":"%s"}\n' \
+printf '{"commit":"%s","version":"%s","source":"%s","build":"%s"}\n' \
   "$(git rev-parse HEAD)" \
   "$(git describe --tags)" \
-  "$(git config --local remote.origin.url | sed -e s,git@github.com:,https://github.com/,)"
+  "$(git config --local remote.origin.url | sed -e s,git@github.com:,https://github.com/,)" \
+  "$CIRCLE_BUILD_URL"