travis: use libgcc-4.8-dev (libgcc-6-dev is not available on Ubuntu 14.04)

Travis image is based on Ubuntu Trusty (14.04), since d83414e1fd we get:

  $ sudo -E \
    apt-get -yq --no-install-suggests --no-install-recommends --force-yes \
      install \
        libaio-dev libattr1-dev libbrlapi-dev libcap-ng-dev libgcc-6-dev \
        libgnutls-dev libgtk-3-dev libiscsi-dev liblttng-ust-dev \
        libncurses5-dev libnfs-dev libnss3-dev libpixman-1-dev libpng12-dev \
        librados-dev libsdl1.2-dev libseccomp-dev libspice-protocol-dev \
        libspice-server-dev libssh2-1-dev liburcu-dev libusb-1.0-0-dev \
        libvte-2.90-dev sparse uuid-dev
  Reading package lists...
  Building dependency tree...
  Reading state information...
  E: Unable to locate package libgcc-6-dev

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

.travis.yml

@@ -13,7 +13,7 @@ addons:
       - libattr1-dev
       - libbrlapi-dev
       - libcap-ng-dev
-      - libgcc-6-dev
+      - libgcc-4.8-dev
       - libgnutls-dev
       - libgtk-3-dev
       - libiscsi-dev