Remove use of function and fix REPO_DIR

Turns out function doesn't work in /bin/sh

And that REPO_DIR was being set to repo/tools instead of repo.
Work with relative paths and mv instead of rm

- Mac OS X doesn't implement ``readlink -f`` and getting absolute path
  of running script in a portal way appears to be horrible.
- Latest mishap with ``rm -rf`` has led me to believe that ``mv``ing the
  directory is somewhat safer than ``rm``.
- Output human-friendly status, don't echo every command.
Merge pull request #193 from alphagov/fix-reinstall-dependencies

Use bash (not sh), fix REPO_DIR to be relative and use mv over rm