Travis: list installed package versions before building

Part of 24629.

.travis.yml

@@ -124,6 +124,10 @@ install:
   - if [[ "$COVERAGE_OPTIONS" != "" ]]; then pip install --user cpp-coveralls; fi
   ## If we're on OSX, and using asciidoc, install asciidoc
   - if [[ "$ASCIIDOC_OPTIONS" == "" ]] && [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install asciidoc; fi
+  ##
+  ## Finally, list installed package versions
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then dpkg-query --show; fi
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew list --versions; fi
 
 script:
   - ./autogen.sh