For Travis CI - Install rake into global gem set.

It was not neccessary before but somehow all the Travis CI build environment for C++ language today do not have rake pre-installed anymore.

.travis.yml

@@ -1,12 +1,14 @@
 language: cpp
 compiler: gcc
 before_install:
-    - sudo add-apt-repository ppa:george-edison55/precise-backports -y
-    - sudo apt-get update -q -y
-    - sudo apt-get install -q -y --no-install-recommends libasound2-dev doxygen graphviz
     - export DISPLAY=:99.0
     - sh -e /etc/init.d/xvfb start
     - /sbin/start-stop-daemon --start --quiet --pidfile /tmp/custom_xvfb_99.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :99 -ac -screen 0 1024x768x24
+    - sudo add-apt-repository ppa:george-edison55/precise-backports -y
+    - sudo apt-get update -q -y
+install:
+    - sudo apt-get install -q -y --no-install-recommends libasound2-dev doxygen graphviz
+    - rvm gemset use global && gem install rake
 script: rake travis_ci
 after_success: rake travis_ci_site_update
 env: