Updated libstemmer_c SHA1 checksum for Sphinx

Hello.

I wasn't able to install/upgrade Sphinx today because of SHA1 mismatch for libstemmer:
```
$ brew upgrade sphinx
==> Upgrading 1 outdated package, with result:
sphinx 2.2.5
==> Upgrading sphinx
==> Downloading http://sphinxsearch.com/files/sphinx-2.2.5-release.tar.gz
Already downloaded: /Library/Caches/Homebrew/sphinx-2.2.5.tar.gz
==> Downloading http://snowball.tartarus.org/dist/libstemmer_c.tgz
######################################################################## 100,0%
Error: SHA1 mismatch
Expected: 9b0f120a68a3c688b2f5a8d0f681620465c29d38
Actual: 1ac6bb16e829e9f3a58f62c27047c26784975aa1
Archive: /Library/Caches/Homebrew/sphinx--stemmer-c.tgz
To retry an incomplete download, remove the file above.
```

There is no news in [libstemmer site](http://snowball.tartarus.org/), so please verify somehow this.

This fix helped me.