Update installation procedure. JBuilder dependency problem

[juansev]

I've installed pageflow using latest code on ruby 2.1.3 and rails 4.18 . Using standard procedure "rails new -d mysql" i was unable to run "bundle install" successfully. Prior to this i copied the extra gem's on the installation text that it is on the homepage.

By default jbuilder 2.X is included on a new application using the following in Gemfile
gem 'jbuilder', '~> 2.0'
This gives an error of a dependency on Pageflow's project.

In order to be able to run "bundle install" i had to change the line to:
gem 'jbuilder', '~> 1.5'

Update the installation procedure with this change or modify pageflow so it is compatible with jbuilder 2.X

[tf]

Hi,

from a quick glance at jbuilder's changelog, 2.0 compatibility should not be a big deal. We should run the pageflow specs with jbuilder 2.0 installed and do a bit manual testing. If all is fine, I see no reason why not to relax the jbuilder dependency in pageflow's gemspec.