.travis.yml: Add "--enable-modules"

We will change the default to "--enable-modules", let's cover it before
the switch.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/.travis.yml b/.travis.yml
index ad66e5b..0ac170b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -98,3 +98,6 @@
           EXTRA_PKGS="liblttng-ust-dev liburcu-dev"
           EXTRA_CONFIG="--enable-trace-backends=ust"
       compiler: gcc
+    - env: TARGETS=i386-softmmu,x86_64-softmmu
+           EXTRA_CONFIG="--enable-modules"
+      compiler: gcc