Remove build_android task from travis temporarily

[Xreki]

fixes #3951

Remove build_android task from travis temporarily to avoid failure on travis because of the cache of third_party. The detail of compiling error is listed as following:

[100%] Built target gtest_main
Install the project...
-- Install configuration: ""
-- Installing: /usr/local/lib/libgmock.a
CMake Error at googlemock/cmake_install.cmake:36 (file):
  file INSTALL cannot copy file
  "/home/travis/build/PaddlePaddle/Paddle/build_android/third_party/gtest/src/extern_gtest-build/googlemock/libgmock.a"
  to "/usr/local/lib/libgmock.a".
Call Stack (most recent call first):
  cmake_install.cmake:37 (include)
make[3]: *** [install] Error 1
make[2]: *** [third_party/gtest/src/extern_gtest-stamp/extern_gtest-install] Error 2
make[1]: *** [CMakeFiles/extern_gtest.dir/all] Error 2
make: *** [all] Error 2
The command "timeout 2580 paddle/scripts/travis/${JOB}.sh # 43min timeout
RESULT=$?; if [ $RESULT -eq 0 ] || [ $RESULT -eq 142 ]; then true; else false; fi;
" exited with 1.
cache.2
store build cache

[lcy-seso]

LGTM.