core: add check of .po files with msgcheck for Travis
core: add install of package libenchant-dev for Travis

Package libenchant-dev is required by pyenchant (used in msgcheck).
core: remove use of virtualenv for Travis
core: add option "APT::Install-Recommends=false" for apt-get (Travis)
core: set option APT::Install-Recommends before commands apt-get / mk-build-deps (Travis)
core: use tee to write in /etc/apt/apt.conf (Travis)
core: use before_script instead of before_install (Travis)
core: add missing semicolon in .travis.yml