Fix codegangsta/cli compile error

codegangsta/cli has added a new value to the Flag struct (to read input
from an environment variable), so these files no longer compile without
error.  This change revises the code in a backwards-compatible way to
compile succesfully with master on codegangsta/cli.
Merge pull request #1 from nathanleclaire/master

Fix codegangsta/cli compile error