tweak wkhtmltoimg to not segfault on x86_64-linux platform

[nyourchuck]

This fixes a problem where wkhtmltoimage-amd64 was segfaulting on x86_64-linux

[chrishein]

Hi, thanks for your contribution.
I'm sorry, but I don't see hoy this fixes a segfault of wkhtmltoimage-amd64 on x86_64-linux.
The proposed change would use the 32 bits executable when the Ruby platform is found to be 64 bits linux.

[nyourchuck]

For whatever reason wkhtmltoimage-amd64 causes a seg fault on our x86_64-linux system, but the 32-bit version still works. I'm not sure how to trouble shoot the binary to see more information.

[chrishein]

I think this is not a solution for the problem.
I would first check to see what RUBY_PLATFORM returns for you ruby VM, see what uname -a shows in you linux command line and also try downloading other versions of the static binaries in https://code.google.com/p/wkhtmltopdf/downloads/list?can=1 and try running them in your environment.

By the way, this gem includes old versions of the static binaries and there have been recent signs of the wkhtmltopdf project coming back to life. Check http://wkhtmltopdf.org/ .