Use travis-ci's multi-os feature.
Set compiler to gcc.
linux: Install libgnome-keyring-dev on travis CI.
Make script quit when error happens in child processes.
linux: Install all necessary deps on CI.
linux: Do "apt-get update" in CI.
linux: Don't set CXX to clang when building node modules.
linux: Start xvfb in CI.
Merge pull request #567 from atom/linux-ci

Add Linux CI