code reduce in Views::Pages::Index#title
tests for ActiveAdmin::Views::Pages::Index#title
Use `sort_by` instead of `sort_by!`

`sort_by!` and other mutator methods are blacklisted as of Ruby on Rails
4.1.0.rc2 and now yield `NoMethodError`.

References:
* https://github.com/rails/rails/commit/d4ee09c
Merge pull request #3064 from jgorset/fix/undefined-method-sort-by!

Use `sort_by` instead of `sort_by!`
Merge pull request #3052 from timoschilling/pages_index_codereduce

code reduce in Views::Pages::Index#title
Update README with info about dependencies for Rails 4.1.
Merge pull request #3066 from sbleon/rails41_install_doc

Update README with info about dependencies for Rails 4.1.
depend on master for certain gems on Rails 4.1

the minitest addition was to get around this error:
```
Warning: you should require 'minitest/autorun' instead.
Warning: or add 'gem "minitest"' before 'require "minitest/autorun"'
`scoped` -> `where(nil)` for Rails 4.1
Rails 4.1 compatibility: ActiveSupport Callbacks

they simply have less public methods for us to use