fix images not being served (closes #928)

[CircleCode]

Note: I am not myself a ruby developer. I only tested the example presentation, and with my setup, images are now served correctly.

[CLAassistant]

All committers have signed the CLA.

[cyberkov]

Hello @CircleCode! I came across this issue today and i can admit that this patch fixes the problem. Would it be possible for you to sign the cla, that hopefully someone with permission to merge could bring this into the gem?

Thank you all very much for your time and effort you put into making my presentations so awesome ;-)

Best wishes, Hannes

[CircleCode]

hey @cyberkov cla signed right now (better late than never ;))