JRuby: make sure we delete keys from the inherited env if set to nil. Closes issue #82.
Bump version to 0.5.4.
Spec environment unset in parent is unset in parent when set in grandparent

MSP-11414

"can unset env vars" checks that with CHILDPROCESS_UNSET=1 in
grandparent (the shell) and parent, the child can unset it by explicitly setting
`process.environment['CHILDPROCESS_UNSET'] = nil`; however, this doesn't
cover that environment variables being unset in the parent, that were
set in the grandparent are unset in the child. "does not see env var
unset in parent" checks this, and it currently fails for JRuby.
Remove environment variables not in ENV that are in ProcessBuilder environment

MSP-11414

Ensure that all key are strings, so there isn't an ordering race to see
if the string or symbol version wins.  Substract keys in merged
enviroment from keys in ProcessBuilder environment to get the keys that
have been removed from ENV and should be removed from ProcessBuilder
environment.