travis: Perform MinGW builds on Fedora 30

Since libvirt-jenkins-ci commit 3c5ac0af41ba, MinGW packages
are installed on Fedora 30 rather than Fedora Rawhide, so we
need to update the Travis CI configuration accordingly.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>

.travis.yml

@@ -33,14 +33,14 @@ matrix:
     - services:
         - docker
       env:
-        - IMAGE="fedora-rawhide"
+        - IMAGE="fedora-30"
         - MINGW="mingw32"
       script:
         - make -f Makefile.ci ci-build@$IMAGE CI_CONFIGURE="$MINGW-configure"
     - services:
         - docker
       env:
-        - IMAGE="fedora-rawhide"
+        - IMAGE="fedora-30"
         - MINGW="mingw64"
       script:
         - make -f Makefile.ci ci-build@$IMAGE CI_CONFIGURE="$MINGW-configure"