diff --git a/.rubocop.yml b/.rubocop.yml
index e86cc3d..144cc61 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -2,7 +2,7 @@ inherit_gem:
   jekyll: .rubocop.yml
 
 AllCops:
-  TargetRubyVersion: 2.1
+  TargetRubyVersion: 2.3
   Exclude:
     - vendor/**/*
     - spec/*.rb
diff --git a/.travis.yml b/.travis.yml
index cdf5bc6..d09341d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -10,10 +10,9 @@ rvm:
   - 2.5
   - 2.4
   - 2.3
-  - 2.2
 env:
   - ""
-  - JEKYLL_VERSION=3.7.0
+  - JEKYLL_VERSION=3.7.3
 matrix:
   include:
     - # GitHub Pages
diff --git a/jekyll-gist.gemspec b/jekyll-gist.gemspec
index b9e7448..579b858 100644
--- a/jekyll-gist.gemspec
+++ b/jekyll-gist.gemspec
@@ -14,7 +14,7 @@ Gem::Specification.new do |spec|
   spec.homepage      = "https://github.com/jekyll/jekyll-gist"
   spec.license       = "MIT"
 
-  spec.required_ruby_version = ">= 2.1"
+  spec.required_ruby_version = ">= 2.3.0"
 
   spec.files         = `git ls-files -z`.split("\x0")
   spec.executables   = spec.files.grep(%r!^bin/!) { |f| File.basename(f) }
@@ -26,6 +26,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "jekyll", ">= 3.0"
   spec.add_development_dependency "rake"
   spec.add_development_dependency "rspec"
-  spec.add_development_dependency "rubocop", "0.51"
+  spec.add_development_dependency "rubocop", "~> 0.57"
   spec.add_development_dependency "webmock"
 end
diff --git a/lib/jekyll-gist/gist_tag.rb b/lib/jekyll-gist/gist_tag.rb
index c265063..fb562a4 100644
--- a/lib/jekyll-gist/gist_tag.rb
+++ b/lib/jekyll-gist/gist_tag.rb
@@ -26,17 +26,18 @@ def render(context)
           script_tag = gist_script_tag(gist_id, filename)
           "#{noscript_tag}#{script_tag}"
         else
-          raise ArgumentError, <<-EOS
-  Syntax error in tag 'gist' while parsing the following markup:
+          raise ArgumentError, <<~ERROR
+          Syntax error in tag 'gist' while parsing the following markup:
 
-    #{@markup}
+          #{@markup}
 
-  Valid syntax:
-    {% gist user/1234567 %}
-    {% gist user/1234567 foo.js %}
-    {% gist 28949e1d5ee2273f9fd3 %}
-    {% gist 28949e1d5ee2273f9fd3 best.md %}
-  EOS
+          Valid syntax:
+            {% gist user/1234567 %}
+            {% gist user/1234567 foo.js %}
+            {% gist 28949e1d5ee2273f9fd3 %}
+            {% gist 28949e1d5ee2273f9fd3 best.md %}
+
+          ERROR
         end
       end
 
diff --git a/lib/jekyll-gist/version.rb b/lib/jekyll-gist/version.rb
index 0de8671..2bef474 100644
--- a/lib/jekyll-gist/version.rb
+++ b/lib/jekyll-gist/version.rb
@@ -2,6 +2,6 @@
 
 module Jekyll
   module Gist
-    VERSION = "1.5.0".freeze
+    VERSION = "1.5.0"
   end
 end