Use "git show" to get the Dockerfile for deferring based on FROM

This gives roughly a full 2x speedup when doing `./build.sh --no-build --all`.
Update several of the docker-library images (buildpack-deps, java, mongo, mysql, node, php, python, rails)
Merge pull request #241 from infosiftr/update-library

Update several of the docker-library images (buildpack-deps, java, mongo, mysql, node, php, python, rails)
Merge pull request #240 from infosiftr/speedup-checking

Use "git show" to get the Dockerfile for deferring based on FROM
Fix "git fetch" lines to work in "detached head" state
Merge pull request #242 from infosiftr/git-fetch

Fix "git fetch" lines to work in "detached head" state