add stm32f4 board to travis build

.travis.yml

@@ -19,3 +19,5 @@ before_script:
 script:
   - make -j2 -C examples/device/cdc_msc_hid BOARD=metro_m0_express
   - make -j2 -C examples/device/cdc_msc_hid BOARD=metro_m4_express
+  - make -j2 -C examples/device/cdc_msc_hid BOARD=stm32f407g_disc1
+