Fix release rake task

[cjcolvar]

The release rake task is broken due to failure loading fcrepo_wrapper, solr_wrapper, and engine_cart. (This is related to #561.) The order of steps is also confusing and leaves things in odd states (a commit for HYDRA_VERSION gets made and included in a tag but not pushed as well as hydra-head's version.rb file gets update but never committed so isn't in the git tag.)