test: used the iptable -I option to insert rule to the head of the filter chain.

Author: zhuizhuhaomeng

.travis.yml

@@ -94,8 +94,8 @@ before_script:
 
 script:
   - sudo iptables -I OUTPUT 1 -p udp --dport 10086 -j REJECT
-  - sudo iptables -A OUTPUT -p tcp --dst 127.0.0.2 --dport 12345 -j DROP
-  - sudo iptables -A OUTPUT -p udp --dst 127.0.0.2 --dport 12345 -j DROP
+  - sudo iptables -I OUTPUT -p tcp --dst 127.0.0.2 --dport 12345 -j DROP
+  - sudo iptables -I OUTPUT -p udp --dst 127.0.0.2 --dport 12345 -j DROP
   - cd luajit2/
   - make -j$JOBS CCDEBUG=-g Q= PREFIX=$LUAJIT_PREFIX CC=$CC XCFLAGS='-DLUA_USE_APICHECK -DLUA_USE_ASSERT -msse4.2' > build.log 2>&1 || (cat build.log && exit 1)
   - sudo make install PREFIX=$LUAJIT_PREFIX > build.log 2>&1 || (cat build.log && exit 1)