Add negated template examples

If a model has 2 float attributes, the auto generated
tests are failing ones because of duplicate value.

Fixing index view tests for a scaffold with multiple float attributes

[skip ci]

Add readme for mailer specs

This is confusing and not useful as documentation, all #738 was intended
to do was to prevent our let definitions from being defined earlier than
3rd party inclusions (and hence being overriden) which was confusing
behaviour, so move it to a spec.

Move #738 regression check to spec

This allows request specs to call assert_select, url_for, etc.

let request specs use integration test services

It doesn't work with Ruby 1.8.7 and 1.9.2.

Don't use Rake v11 with older rubies