Update Linux binary to version 1.8.13

[RobinvanderVliet]

wget "http://ftp.nl.debian.org/debian/pool/main/p/pngcrush/pngcrush_1.8.13-0.1_amd64.deb" -q -O "pngcrush.deb"

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sha1sum pngcrush.deb

0368fe4b52dbb1b6ca75793599b6f2bf5f72f53f pngcrush.deb

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dpkg-deb -xv pngcrush.deb .

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./usr/bin/pngcrush -version

pngcrush 1.8.13, uses libpng 1.6.36 and zlib 1.2.11
Check http://pmt.sf.net/
for the most recent version.

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sha1sum ./usr/bin/pngcrush

91fc93611e63bbbc82791be343f64239be789293 ./usr/bin/pngcrush

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I have committed the file ./usr/bin/pngcrush. If you repeat the previous steps, you should see the same sha1 hashes.

Fixes #30!