travis.yml: Export CC and CXX inside the docker instance

docker does not inherit the parent environment and loses the CC/CXX
settings for testing against clang and gcc.

.travis.yml

@@ -36,5 +36,5 @@ script:
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then echo FROM jpakkane/mesonci:yakkety > Dockerfile; fi
   - if [[ "$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 && TRAVIS=true ./run_tests.py -- $MESON_ARGS"; fi
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then docker run withgit /bin/sh -c "cd /root && TRAVIS=true CC=$CC CXX=$CXX ./run_tests.py -- $MESON_ARGS"; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then SDKROOT=$(xcodebuild -version -sdk macosx Path) ./run_tests.py --backend=ninja -- $MESON_ARGS ; fi