typo sha1 -> sha256

@alebcay here is a drawback to using conditionals:

Because my testing machine was running Mavericks, the error in the `else` block was not seen by `audit` or even `install`.

I suppose we could rewrite the test suite to audit each cask with overrides on a matrix of permissible conditional values.  But: slow, fragile, extra maintenance work.

@phinze is right, we should develop a first-class way of handling OS and hardware variants in the DSL.   Then the test suite could ask the cask: "hey, what are your variants?"

For the moment, conditionals are only used in 15 Casks.