travis: Also set OBJC and OBJCXX to CC/CXX

Else they are only autodetected and we don't test both GCC and Clang.

.travis.yml

@@ -40,5 +40,5 @@ 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 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 -- $MESON_ARGS ; fi
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then docker run withgit /bin/sh -c "cd /root && TRAVIS=true CC=$CC CXX=$CXX OBJC=$CC OBJCXX=$CXX ./run_tests.py -- $MESON_ARGS"; fi
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then SDKROOT=$(xcodebuild -version -sdk macosx Path) OBJC=$CC OBJCXX=$CXX ./run_tests.py --backend=ninja -- $MESON_ARGS ; fi