fix rails 4 deprecation warnings

@pwiebe

Fix following deprecation messages:

DEPRECATION WARNING: Relation#calculate with finder options is deprecated. Please build a scope and then call calculate on it instead. (called from run at /bundler/gems/rails-simple-search-033c6181bba0/lib/rails-simple-search.rb:61)
DEPRECATION WARNING: The :distinct option for `Relation#count` is deprecated. Please use `Relation#distinct` instead. (eg. `relation.distinct.count`). (called from run at /bundler/gems/rails-simple-search-033c6181bba0/lib/rails-simple-search.rb:61)
DEPRECATION WARNING: Relation#all is deprecated. If you want to eager-load a relation, you can call #load (e.g. `Post.where(published: true).load`). If you want to get an array of records from a relation, you can call #to_a (e.g. `Post.where(published: true).to_a`). (called from run at /bundler/gems/rails-simple-search-033c6181bba0/lib/rails-simple-search.rb:79)