don't overide SHELL in env. unless it is explicitely requested

@schaal 

Hi, I saw in [man:systemd.exec(5)](http://www.freedesktop.org/software/systemd/man/systemd.exec.html#Environment%20variables%20in%20spawned%20processes)
that some variables are allways set by systemd itself, so it's not needed to set them in the generated service files .

This little change would only set this environement variable if explicitely requested in a crontab.

This amends this:
https://github.com/kstep/systemd-crontab-generator/commit/ff6968ace6ba254dbeb76beea89f4a4b386689d2