travis: run "make distcheck" instead of just "make check"

Running "make distcheck" includes the "make check", and "make dist"
targets. It ensures that we have CLEANFILES and uninstall rules setup
correctly, as well as validating VPATH builds succeed.
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

.travis.yml

@@ -96,7 +96,7 @@ before_script:
 script:
   # Many unit tests still fail on macOS, and there are a bunch of issues with
   # syntax-check as well, so skip those steps on that platform for now
-  - make -j3 && if [ "$TRAVIS_OS_NAME" != "osx" ]; then make -j3 syntax-check && make -j3 check; fi
+  - make -j3 && if [ "$TRAVIS_OS_NAME" != "osx" ]; then make -j3 syntax-check && make -j3 distcheck; fi
 
 after_failure:
   - echo '============================================================================'