For Travis CI - use 'travis_retry' command to improve the odd.

See https://github.com/travis-ci/travis-ci/issues/2000.
[ci skip]

.travis.yml

@@ -48,7 +48,7 @@ before_install:
     - sudo apt-get update -q -y
 install:
     - sudo apt-get install -q -y --no-install-recommends libasound2-dev
-    - rvm gemset use global && gem install rake
+    - rvm gemset use global && travis_retry gem install rake
     - bash -c "[ x$ENABLE_64BIT == 'x' ]" && sudo apt-get remove -q -y gvfs-daemons && sudo apt-get install -q -y libxrandr-dev:i386 libgl1-mesa-dev:i386 libxext-dev:i386 libxrender-dev:i386 g++-multilib && export CMAKE_PREFIX_PATH=/usr/lib/i386-linux-gnu || true
     - bash -c "( [ $SITE_UPDATE_ON_MASTER_COMMIT ] || [ $PACKAGE_UPLOAD ] )" && sudo apt-get install -q -y --no-install-recommends doxygen graphviz || true
     - bash -c "[ $WINDOWS ]" && sudo apt-get install -q -y gcc-mingw-w64 gcc-mingw-w64-i686 gcc-mingw-w64-x86-64 g++-mingw-w64 g++-mingw-w64-i686 g++-mingw-w64-x86-64 binutils-mingw-w64 binutils-mingw-w64-i686 binutils-mingw-w64-x86-64 && export MINGW_PREFIX=/usr/bin/x86_64-w64 MINGW_ROOT=/usr/x86_64-w64-mingw32 || true