Fix pom with non-default :default profile

Unmerge expanded its profiles based on the profiles in the project map (which
don't include :default), rather than profiles in the
:profiles metadata.

The :default keys were also hard coded in the pom generation, so ignored any
additions.

This is in support of https://github.com/lynaghk/cljx/pull/48

Note the reordering of the dependencies in the generated pom.  Is this
significant or not?  The order was :dev :test :base, and is now :base
:dev :test.