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
linux_deps.sh: buildVer=0.7.0
linux_deps.sh: Creating $node_webkit_tarball.sha1
linux_deps.sh: Improve log messages etc.
linux_deps.sh: buildVer=0.7.1
linux_deps.sh: Remove binaries from previous LT release during 1st build
of a new LT release