[homebrew] Update notmuch to version 0.18 (#29497)

[David Bremner]

"Wael M. Nasreddine" <wael.nasreddine at gmail.com> writes:

> That's what it requests in ./configure anyway.
>
> @David does notmuch work with zlib 1.2.5.0 or does it really need 1.2.5.2?
> On May 22, 2014 1:08 PM, "Jack Nagel" <notifications at github.com> wrote:
>
>> OS X has zlib 1.2.5, so I'd like to find out if notmuch really, truly
>> depends on 1.2.5.2.
>>

Yes, we really need 1.2.5.2, for the reason documented in INSTALL.

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