Add Travis test for DUAL_X_CARRIAGE

.travis.yml

   - opt_enable EEPROM_SETTINGS EEPROM_CHITCHAT
   - build_marlin
   #
+  # Test DUAL_X_CARRIAGE
+  #
+  - restore_configs
+  - opt_set MOTHERBOARD BOARD_RAMPS_14_EEB
+  - opt_set EXTRUDERS 2
+  - opt_set TEMP_SENSOR_1 1
+  - opt_enable USE_XMAX_PLUG
+  - opt_enable_adv DUAL_X_CARRIAGE
+  - build_marlin
+  #
   ### LCDS ###
   #
   #