Add "passphrase" to filtered parameters

In local development, I could see my passphrase in the list Rails parameters
in plain text. This is because only the key `:password` was filtered. This
commit filters both `:password` and `:passphrase` to prevent passwords from
being logged in plain text.
Merge pull request #102 from sethvargo/sethvargo/filter_params

Add "passphrase" to filtered parameters