autoconf on el6 fails

[lkarsten]

[lkarsten@el6 libvmod-example]$ ./autogen.sh 
++ pkg-config --variable=datarootdir varnishapi
+ dataroot=/usr/share
+ aclocal -I m4 -I /usr/share/aclocal
configure.ac:1: error: Autoconf version 2.64 or higher is required
configure.ac:1: the top level
autom4te: /usr/bin/m4 failed with exit status: 63
aclocal: autom4te failed with exit status: 63

[nigoroll]

@fgsch pointed out in #18 that this smells like requiring the same fix as varnish/Varnish-Cache@ecfc757 but as this was applied to varnish.m4, we should not require it in libvmod-example.
So I'd presume all that needs to be done is to set AC_PREREQ(2.59) as in varnish configure.ac