Fix erroneous shell command form, fixes #1683.
Set env var if .lein-fast-trampoline exists.

Makes it possible to just use LEIN_FAST_TRAMPOLINE in the Lein process,
to check if the user is using fast trampolining. Strictly speaking, one
previously had to check whether LEIN_FAST_TRAMPOLINE was set or check
that .lein-fast-trampoline existed inside Lein.
Merge pull request #1691 from technomancy/fix-erroneous-shell-command-form

Fix negation in shell-command form.