Allow failures for all mongoid 4.0 specs

It looks like #with option doesn't work right now. Since mongoid 4.0 is not
released yet, just allow them to fail for now. We'll come back later.
Do not include railties and rspec-rails gem [ci skip]

They are unneeded if Sinatra is used.
Test Kaminari::ActionViewExtension only when Rails is defined
Revert "Use SVG for status badges in README"

This reverts commit d1be2150f098abdb261b0cd752499c582b1f4262 because the badge
for travis doesn't show up correctly.