Update .travis.yml to install dependencies for virtualbox

Author: mapuri

.travis.yml

@@ -7,5 +7,7 @@ before_install:
   - sudo apt-get update
   - wget https://dl.bintray.com/mitchellh/vagrant/vagrant_1.7.2_x86_64.deb
   - sudo dpkg -i vagrant_*.deb
+  - wget http://download.openvz.org/kernel/branches/rhel6-2.6.32/042stab090.5/linux-headers-2.6.32-openvz-042stab090.5-amd64_1_amd64.deb
+  - sudo dpkg -i linux-headers-2.6.32-openvz-042stab090.5-amd64_1_amd64.deb
   - sudo apt-get -f install
   - sudo apt-get install -y virtualbox