Doctor check for JENV_LOADED environment variable

* Users may not have jenv init in their shell profile, but may load it
* indirectly in another manner, so check for JENV_LOADED=1 in
* environment.

* Fix misspelling of "correctly"
Merge pull request #58 from schlosna/hotfix/jenv-loaded-doctor

Doctor check for JENV_LOADED environment variable