.travis.yml: Add aarch64-* targets

Now the AArch64 targets are in mainline we can include them in our
Travis test matrix.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

.travis.yml

@@ -16,6 +16,7 @@ env:
   matrix:
   - TARGETS=alpha-softmmu,alpha-linux-user
   - TARGETS=arm-softmmu,arm-linux-user
+  - TARGETS=aarch64-softmmu,aarch64-linux-user
   - TARGETS=cris-softmmu
   - TARGETS=i386-softmmu,x86_64-softmmu
   - TARGETS=lm32-softmmu