Feature: Don't show the decrypted setting when pushing to Heroku

This just makes things generally more secure when dealing with log output on
things like CI servers.  If you want to see the values that would be pushed, use
`chamber show --as-env --only-secure`, etc.

Closes #31

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Change-Id:     If4ff1bf49c10e4d85f548c094e72aefad40f6863
Signed-off-by: Jeff Felchner <accounts+git@thekompanee.com>