The gem wkhtmltopdf and pdfkit in a Rails app on Heroku produces an error

I have a rails 3.2 app on heroku and these 2 gems:

ruby "2.2.4"
gem 'rails', '3.2.16'

# .......
gem 'wkhtmltopdf-binary-11'
gem 'pdfkit'

However, when I'm trying to generate pdf, I get the following errors:

QSslSocket: cannot resolve SSLv2_client_method
QPixmap: Cannot create a QPixmap when no GUI is being used

Obviously, I can't install a library or update it on heroku to fix the errors. How to fix them? And the same kind of error I have locally, but I'm on Arch.