CI: allow travis to upload failed test log on server

.travis.yml

@@ -52,6 +52,7 @@ env:
     # The next declaration is the encrypted COVERITY_SCAN_TOKEN, created
     #   via the "travis encrypt" command using the project repo's public key
     - secure: "FjIwqZQV2FhNPWYITX5LZXTE38yYqBaQdbm3QmbEg/30wnPTm1ZOLIU7o/aSvX615ImR8kHoryvFPDQDWc6wWfqTEs3Ytq2kIvcIJS2Y5l/0PFfpWJoH5gRd6hDThnoi+1oVMLvj1+bhn4yFlCCQ2vT/jxoGfiQqqgvHtv4fLzI="
+    - AUTOTEST_UPLOAD=true  # Upload logs of failed tests to ArduPilot server; requires authentication
 
 matrix:
   fast_finish: true