Add bin/lein-pkg to the list of files that get version substitutions.

Currently lein-pkg is forgotten about, which means it is always pointing to
the 2.0.0-SNAPSHOT version. This means packagers much edit this file
themselves.
Merge pull request #936 from metajack/fix-release-for-lein-pkg

Add bin/lein-pkg to the list of files that get version substitutions.