Update .travis.yml

use lwip2.0 low memory to pass travis

Author: Luc

.travis.yml

@@ -21,6 +21,9 @@ before_script:
   - echo 'build.flash_size=4M' >> platform.txt
   - echo 'build.flash_mode=dio' >> platform.txt
   - echo 'build.f_cpu=160000000L' >> platform.txt
+  - sed -i "s/build.lwip_lib=-llwip_gcc/build.lwip_lib=-llwip2-536-feat/g" ./platform.txt
+  - sed -i "s/build.lwip_include=lwip/build.lwip_include=lwip2/g" ./platform.txt
+  - sed -i "s/build.lwip_flags=-DLWIP_OPEN_SRC/build.lwip_flags=-DLWIP_OPEN_SRC -DTCP_MSS=536 -DLWIP_FEATURES=1 -DLWIP_IPV6=0/g" ./platform.txt
   - sed -i "s/generic.build.f_cpu=80000000L/generic.build.f_cpu=160000000L/g" ./boards.txt
   - rm -fr ./libraries/ESP8266mDNS/src/LEAmDNS.cpp
   - cd $HOME/arduino_ide/hardware