add a 'flow' enum to approval_groups
add SimpleEnum for Rails 4 compatability
clean up approval group form indentation
Merge remote-tracking branch 'origin/master' into 79249190_linear_group

Conflicts:
	Gemfile
	Gemfile.lock
add form option for selecting linear flow
use built-in enum support form rails 4.1
validate presence of ApprovalGroup#flow
simplify select options helper
feature flag the flow
add without_feature spec helper
store approval group flow as a string
Merge remote-tracking branch 'origin/master' into 79249190_linear_group

Conflicts:
	spec/integration/cart_creation_navigator_spec.rb
	spec/integration/cart_creation_spec.rb
add spec around validation for ApprovalGroup#flow
Merge pull request #84 from 18F/79249190_linear_group

[#79249190] set up linear group