Use Bundler.setup instead of require

If we are not autoloading our code, external libraries
shouldn't be autoloaded anyway.
See also http://anti-pattern.com/use-bundler-setup-instead-of-bundler-
require
Require sequel in initializer
Require pliny/utils in initializer
Merge pull request #64 from cvut/bundler_setup

Use Bundler.setup instead of .require