Properly install rubygems/activerecord on all platforms

On our debian systems, we don't have rubygems, so we need to install it.
Also, EL5 has Ruby 1.8.5, and thus the activerecord gem won't install,
so we have to use yum to get the version from yum.puppetlabs.com.
Add a `bounded-pmap` function, and use it in the export tool

This commit adds a new namespace called `concurrent`, intended to
contain some utility functions relating to concurrency.  (I expect
that in most cases these will simply be thin wrappers around
clojure concurrency functions in combination with structures from
`java.util.concurrent`.)

The first major function in this namespace is `bounded-pmap`, which
is just like clojure's `pmap`, but allows you to pass in an integer
value `max-threads` and prevents concurrent evaluation of more than
`max-threads` items in the resulting sequence.

Also ports the `export` command-line tool over to use `bounded-pmap`,
which improves performance by about 3x and drastically reduces
memory consumption.