Update ruby_parser to eliminate Ruby 2.7 warning messages

Running `bin/rake gettext:regenerate` would spew:

```
NOTE: RubyParser::V27 undefined, using RubyParser::V26.
```

This commit updates the ruby_parser to v3.15.0, which brings in Ruby 2.7
support: https://github.com/seattlerb/ruby_parser/commit/b6fc793

Gemfile

@@ -317,7 +317,7 @@ gem 'premailer-rails', '~> 1.10.3'
 gem 'gitlab-labkit', '0.13.1'
 
 # I18n
-gem 'ruby_parser', '~> 3.8', require: false
+gem 'ruby_parser', '~> 3.15', require: false
 gem 'rails-i18n', '~> 6.0'
 gem 'gettext_i18n_rails', '~> 1.8.0'
 gem 'gettext_i18n_rails_js', '~> 1.3'

Gemfile.lock

@@ -1042,7 +1042,7 @@ GEM
       nokogiri (>= 1.5.10)
     ruby-statistics (2.1.2)
     ruby2_keywords (0.0.2)
-    ruby_parser (3.13.1)
+    ruby_parser (3.15.0)
       sexp_processor (~> 4.9)
     rubyntlm (0.6.2)
     rubypants (0.2.0)
@@ -1085,7 +1085,7 @@ GEM
     sentry-raven (3.0.4)
       faraday (>= 1.0)
     settingslogic (2.0.9)
-    sexp_processor (4.12.0)
+    sexp_processor (4.15.1)
     shellany (0.0.1)
     shoulda-matchers (4.0.1)
       activesupport (>= 4.2.0)
@@ -1474,7 +1474,7 @@ DEPENDENCIES
   ruby-fogbugz (~> 0.2.1)
   ruby-prof (~> 1.3.0)
   ruby-progressbar
-  ruby_parser (~> 3.8)
+  ruby_parser (~> 3.15)
   rubyzip (~> 2.0.0)
   rugged (~> 0.28)
   sanitize (~> 5.2.1)