Add recommended workaround for ESP8266 core 2.5.0-beta3 (#606)

Per esp8266/Arduino#5678

Hopefully should fix travis builds.

Author: crankyoldgit

.travis.yml

@@ -1,7 +1,7 @@
 language: c
 env:
-  - BD=esp8266:esp8266:nodemcuv2:xtal=80,eesz=4M3M
-  - BD=esp8266:esp8266:d1_mini:xtal=80,eesz=4M3M
+  - BD=esp8266:esp8266:nodemcuv2:xtal=80,eesz=4M3M,ip=lm2f,exception=disabled
+  - BD=esp8266:esp8266:d1_mini:xtal=80,eesz=4M3M,ip=lm2f,exception=disabled
 before_install:
   - "/sbin/start-stop-daemon --start --quiet --pidfile /tmp/custom_xvfb_1.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :1 -ac -screen 0 1280x1024x16"
   - sleep 3