linux_deps.sh: If 'which curl' fails, exit with its return code.
linux_deps.sh: re-use downloaded LightTableLinux*.gz, clojure-plugin

* download LightTableLinux*.tar.gz only if it doesn't exist and always
  verify its sha1 checksum.
* store clojure-plugin as a git-submodule and clone it only once. Then
  update it with git pull --rebase
linux_deps.sh: buildVer=0.6.7