travis: Manually export TRAVIS=true for linux builds

Docker wipes the environment before starting a build, so TRAVIS doesn't
get set and our run_tests.py script doesn't print logs for failing
tests.

.travis.yml

@@ -22,5 +22,5 @@ script:
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then echo FROM jpakkane/mesonci:xenial > Dockerfile; fi
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then echo ADD . /root >> Dockerfile; fi
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then docker build -t withgit .; fi
-  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then docker run withgit /bin/sh -c "cd /root && ./run_tests.py"; fi
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then docker run withgit /bin/sh -c "cd /root && TRAVIS=true ./run_tests.py"; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then SDKROOT=$(xcodebuild -version -sdk macosx Path) ./run_tests.py --backend=ninja ; fi