Update ruby-will-paginate to 3.1.5.

3.1.5

    * Avoid Rails 5 deprecation warning about original_exception
    * No longer generates <a rel="start"> for first page link since "start"
      is an invalid rel value

3.1.4 (seems not released)

3.1.3

    * Rails 5 compatibility for inheriting query params in pagination links
    * Fix locale load path so that application can override will_paginate's
      built-in translations.

3.1.2

    * Further fix security vulnerability when generating pagination URLs in
      Rails.

3.1.1

    * Fixed security vulnerability when generating pagination URLs in Rails.

3.1.0

    * Rails 5.0 support

    * Drop support for old Active Record finder params to paginate():

	# now unsupported:
	User.paginate(:page => 1, :order => 'created_at DESC', :conditions => ...)

	# convert to Arel syntax instead:
	User.order('created_at DESC').where(...).paginate(:page => 1)
	# or, even better:
	User.order('created_at DESC').where(...).page(1)

    * Add support for pagination with Mongoid


(taca)