Replace Rubocop with standard

Following an [update to the thoughtbot guide][guides PR], this commit
removes Rubocop and replaces it with [standard].

[guides PR]: thoughtbot/guides#606
[standard]: https://github.com/testdouble/standard

Gemfile

@@ -7,5 +7,4 @@ gem "aruba"
 gem "cucumber", "1.3.19"
 gem "rake"
 gem "rspec-rails"
-
-gem "rubocop", "0.54", require: false
+gem "standard"

Gemfile.lock

@@ -76,7 +76,6 @@ GEM
     parallel (1.19.1)
     parser (2.7.1.3)
       ast (~> 2.4.0)
-    powerpack (0.1.2)
     rack (2.2.2)
     rack-test (1.1.0)
       rack (>= 1.0, < 3)
@@ -93,6 +92,8 @@ GEM
       thor (>= 0.20.3, < 2.0)
     rainbow (3.0.0)
     rake (13.0.1)
+    regexp_parser (1.7.0)
+    rexml (3.2.4)
     rspec-core (3.9.2)
       rspec-support (~> 3.9.3)
     rspec-expectations (3.9.2)
@@ -110,14 +111,23 @@ GEM
       rspec-mocks (~> 3.9)
       rspec-support (~> 3.9)
     rspec-support (3.9.3)
-    rubocop (0.54.0)
+    rubocop (0.85.0)
       parallel (~> 1.10)
-      parser (>= 2.5)
-      powerpack (~> 0.1)
+      parser (>= 2.7.0.1)
       rainbow (>= 2.2.2, < 4.0)
+      regexp_parser (>= 1.7)
+      rexml
+      rubocop-ast (>= 0.0.3)
       ruby-progressbar (~> 1.7)
-      unicode-display_width (~> 1.0, >= 1.0.1)
+      unicode-display_width (>= 1.4.0, < 2.0)
+    rubocop-ast (0.0.3)
+      parser (>= 2.7.0.1)
+    rubocop-performance (1.6.1)
+      rubocop (>= 0.71.0)
     ruby-progressbar (1.10.1)
+    standard (0.4.7)
+      rubocop (~> 0.85.0)
+      rubocop-performance (~> 1.6.0)
     thor (1.0.1)
     thread_safe (0.3.6)
     thread_safe (0.3.6-java)
@@ -137,7 +147,7 @@ DEPENDENCIES
   factory_bot_rails!
   rake
   rspec-rails
-  rubocop (= 0.54)
+  standard
 
 BUNDLED WITH
    2.1.4

gemfiles/rails5.0.gemfile

@@ -7,7 +7,7 @@ gem "aruba"
 gem "cucumber", "1.3.19"
 gem "rake"
 gem "rspec-rails"
-gem "rubocop", "0.54", require: false
+gem "standard"
 gem "byebug"
 gem "listen", "~> 3.0.5"
 gem "puma", "~> 3.0"

gemfiles/rails5.1.gemfile

@@ -7,7 +7,7 @@ gem "aruba"
 gem "cucumber", "1.3.19"
 gem "rake"
 gem "rspec-rails"
-gem "rubocop", "0.54", require: false
+gem "standard"
 gem "byebug"
 gem "listen", ">= 3.0.5", "< 3.2"
 gem "puma", "~> 3.7"

gemfiles/rails5.2.gemfile

@@ -7,7 +7,7 @@ gem "aruba"
 gem "cucumber", "1.3.19"
 gem "rake"
 gem "rspec-rails"
-gem "rubocop", "0.54", require: false
+gem "standard"
 gem "byebug"
 gem "listen", ">= 3.0.5", "< 3.2"
 gem "puma", "~> 3.11"

gemfiles/rails6.0.gemfile

@@ -7,7 +7,7 @@ gem "aruba"
 gem "cucumber", "1.3.19"
 gem "rake"
 gem "rspec-rails"
-gem "rubocop", "0.54", require: false
+gem "standard"
 gem "byebug"
 gem "listen", "~> 3.2"
 gem "puma", "~> 4.1"