[Fix #112] Can now update status of existing post.

Bug was in support method `PostsController#select_updates`, which had
not been updated since publication status was introduced. Oops.

RSpec: 812 examples, 0 failures (but see Issue #87)
RuboCop: 152 files inspected, no offenses detected
Merge pull request #113 from jdickey/issue-112

[Fix #112] Can now update status of existing post.