Remove description of the dependencies on Mac OS X

According to the documentation
(https://developers.google.com/speed/webp/docs/precompiled#getting_cwebp
_dwebp_and_the_webp_libraries):

> Conversely, the libpng, libjpeg and libtiff libraries have been
statically linked in the Mac OS X binaries. They do not require
additional installation.
Merge pull request #2 from shinnn/master

Remove description of the dependencies on Mac OS X