Fix handling the -n / --no-pull switch.

Commit ea14d2eedbb4b3fbc50750f3a66aafc7b773781d introduced a typo which
clobbered :no_pull with an unrelated command line switch, so fix that.

While at it, make -n work as well, not just the full --no-pull version.
Merge pull request #71 from wulczer/fix-no-pull-cmdline-switch

Fix handling the -n / --no-pull switch.