Replacing regular expression with string comparison

Based on benchmarking, string comparison is faster than regular
express. I also believe that `end_with?(".")` reads better than
mentally parsing a regular expression; It is certainly more "new
developer" friendly.

Here is the benchmark that I ran locally to test performance:

```ruby
require 'benchmark'
n = 30_000_000
Benchmark.bm do |x|
  x.report { n.times { ".".end_with?(".") } }
  x.report { n.times { "." =~ /\.$/ } }
end

```

And here are the results:

```log
    user     system      total        real
4.108148   0.021553   4.129701 (  4.194707)
7.508562   0.018433   7.526995 (  7.585707)
```

And as a bonus, when attempting to bump Rubocop's `TargetRubyVersion`
from `2.3` to `2.4`, the one violation was that we were using `=~`.
Rubocop recommended that we switch to `match?` for regular
expressions.

app/helpers/hyrax/citations_behaviors/formatters/apa_formatter.rb

@@ -41,7 +41,7 @@ def format_authors(authors_list = [])
               text << ", " << author
             end
           end
-          text << "." unless text =~ /\.$/
+          text << "." unless text.end_with?(".")
           text
         end
 

app/helpers/hyrax/citations_behaviors/formatters/chicago_formatter.rb

@@ -35,7 +35,7 @@ def format_authors(authors_list = [])
           text << " et al." if authors_list.length > 7
           # if for some reason the first author ended with a comma
           text.gsub!(',,', ',')
-          text << "." unless text =~ /\.$/
+          text << "." unless text.end_with?(".")
           whitewash(text)
         end
         # rubocop:enable Metrics/MethodLength
@@ -45,7 +45,7 @@ def format_date(pub_date); end
         def format_title(title_info)
           return "" if title_info.blank?
           title_text = chicago_citation_title(title_info)
-          title_text << '.' unless title_text =~ /\.$/
+          title_text << '.' unless title_text.end_with?(".")
           title_text = whitewash(title_text)
           " <i class=\"citation-title\">#{title_text}</i>"
         end

app/helpers/hyrax/citations_behaviors/formatters/mla_formatter.rb

@@ -27,7 +27,7 @@ def format_authors(authors_list = [])
           authors_list = Array.wrap(authors_list)
           text = concatenate_authors_from(authors_list)
           if text.present?
-            text << "." unless text =~ /\.$/
+            text << "." unless text.end_with?(".")
             text << " "
           end
           text