Removed requirement that is not needed

I have removed libsodium from CentOS system requirements, as it is
part PyNaCl.

Signed-off-by: Richard Kellner <richard.kellner@gmail.com>

README.rst

@@ -111,7 +111,7 @@ Installing prerequisites on ArchLinux::
 Installing prerequisites on CentOS 7::
 
     sudo yum install libtiff-devel libjpeg-devel libzip-devel freetype-devel \
-                     lcms2-devel libwebp-devel libsodium tcl-devel tk-devel \
+                     lcms2-devel libwebp-devel tcl-devel tk-devel \
                      python-virtualenv libffi-devel openssl-devel
     sudo yum groupinstall "Development Tools"