Address linter issues

lib/mjml.rb

@@ -76,6 +76,7 @@ def self.check_for_custom_mjml_binary
   def self.check_for_mjml_package(package_manager, bin_command, binary_path)
     stdout, _, status = Open3.capture3("which #{package_manager}")
     return unless status.success?
+
     pm_bin = stdout.chomp
 
     stdout, _, status = Open3.capture3("#{pm_bin} #{bin_command}")
@@ -94,14 +95,16 @@ def self.check_for_package_mjml_binary
   end
 
   def self.check_for_global_mjml_binary
-    stdout, _, status = Open3.capture3("which mjml")
+    stdout, _, status = Open3.capture3('which mjml')
     return unless status.success?
+
     mjml_bin = stdout.chomp
     return mjml_bin if mjml_bin.present? && check_version(mjml_bin)
   end
 
   def self.check_for_mrml_binary
     return unless Mjml.use_mrml
+
     MRML.present?
   rescue NameError
     Mjml.mjml_binary_error_string = 'Couldn\'t find MRML - did you add \'mrml\' to your Gemfile?'

test/mjml_test.rb

@@ -173,14 +173,14 @@ class NotifierMailerTest < ActiveSupport::TestCase
       expect(Mjml.mjml_binary_error_string).must_equal 'Couldn\'t find MRML - did you add \'mrml\' to your Gemfile?'
     end
 
-    it "with nothing on the path, sets valid_mjml_binary to nil" do
-      old_path = ENV["PATH"]
-      ENV["PATH"] = "./test/fixtures/valid-mjml-binary/empty-path/"
+    it 'with nothing on the path, sets valid_mjml_binary to nil' do
+      old_path = ENV['PATH']
+      ENV['PATH'] = './test/fixtures/valid-mjml-binary/empty-path/'
 
       Mjml.stubs(:puts) # silence printed error message from test output
       assert_nil(Mjml.valid_mjml_binary)
     ensure
-      ENV["PATH"] = old_path
+      ENV['PATH'] = old_path
     end
   end
 end