Fix rubocop config and cleanup offenses

[Matt Lindsey]

Running bundle exec rubycop currently gives a config error.

Fix by replacing the following line in .rubocop.yml:
Gemspec/DateAssignment: # (new in 1.10)
With:
Gemspec/DeprecatedAttributeAssignment:

Also add following line to enable rubocop-rails (which is already in Gemfile):
require: rubocop-rails

Then try to correct the offenses given when you run rubocop:
433 files inspected, 1282 offenses detected, 471 offenses autocorrectable