Change the default completion system to completing-read

Also remove the recommendation of Grizzl, and re-work the documentation
of the completion system choices, to accurately present the advantages
and disadvantages of each system and allow the user to make an educated
choice by themselves.
[Fix #301] Start a deferred check when Flycheck is running

When Flycheck is already running while we process a new automatic check
event, the results of the current syntax check will most likely be
outdated when it's finished, so we start a new deferred check to
immediately recheck the buffer after the current check finished.
Remove use of deprecated RuboCop flag --silent

`--silent` has been out of commission since RuboCop 0.12.
Merge pull request #304 from bbatsov/rubocop-silent

Remove use of deprecated RuboCop flag --silent
Run deferred checks after the syntax check finished [#301]
Update changelog [#301]