TravisCI: use "travis_retry" for network dependent cmds.

travis_retry: http://blog.travis-ci.com/2013-05-20-network-timeouts-build-retries/

.travis.yml

@@ -23,9 +23,10 @@ matrix:
     - env: TARGET=flash8
 
 before_install:
-  - sudo apt-get update
-  - sudo apt-get install ocaml zlib1g-dev libgc-dev -y
-  - git clone https://github.com/HaxeFoundation/neko.git ~/neko && cd ~/neko && make && sudo make install && cd $TRAVIS_BUILD_DIR
+  - travis_retry sudo apt-get update
+  - travis_retry sudo apt-get install ocaml zlib1g-dev libgc-dev -y
+  - travis_retry git clone https://github.com/HaxeFoundation/neko.git ~/neko
+  - cd ~/neko && make && sudo make install && cd $TRAVIS_BUILD_DIR
 
 script:
   - make