[ci] flag the documentation build bot

Author: gnzlbg

.travis.yml

@@ -15,7 +15,8 @@ matrix:
     - os: osx
       env: TARGET=x86_64-apple-darwin NO_ADD=1
       script: ci/run.sh
-    - install: true
+    - env: DOCUMENTATION
+      install: true
       script: ci/dox.sh
     - env: RUSTFMT=On TARGET=x86_64-unknown-linux-gnu NO_ADD=1
       script: |