[FIX] report_xls dependency

.travis.yml

@@ -10,6 +10,7 @@ virtualenv:
   system_site_packages: true
 
 install:
+  - git clone https://github.com/OCA/reporting-engine ${HOME}/reporting-engine -b ${VERSION}
   - git clone https://github.com/OCA/maintainer-quality-tools.git ${HOME}/maintainer-quality-tools
   - export PATH=${HOME}/maintainer-quality-tools/travis:${PATH}
   - travis_install_nightly