OJT21-52: Add pre-commit and edit .ruby-version

+repos:
+- repo: https://github.com/rubocop/rubocop
+  rev: v1.8.1
+  hooks:
+    - id: rubocop
\ No newline at end of file

-2.7.3
+2.6.7

@@ -60,6 +60,7 @@ GEM
       minitest (>= 5.1)
       tzinfo (~> 2.0)
       zeitwerk (~> 2.3)
+    ast (2.4.2)
     bootsnap (1.7.6)
       msgpack (~> 1.0)
     builder (3.2.4)
@@ -91,6 +92,9 @@ GEM
     nokogiri (1.11.7)
       mini_portile2 (~> 2.5.0)
       racc (~> 1.4)
+    parallel (1.20.1)
+    parser (3.0.2.0)
+      ast (~> 2.4.1)
     puma (5.4.0)
       nio4r (~> 2.0)
     racc (1.5.2)
@@ -123,10 +127,25 @@ GEM
       method_source
       rake (>= 0.13)
       thor (~> 1.0)
+    rainbow (3.0.0)
     rake (13.0.6)
     rb-fsevent (0.11.0)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
+    regexp_parser (2.1.1)
+    rexml (3.2.5)
+    rubocop (1.18.4)
+      parallel (~> 1.10)
+      parser (>= 3.0.0.0)
+      rainbow (>= 2.2.2, < 4.0)
+      regexp_parser (>= 1.8, < 3.0)
+      rexml
+      rubocop-ast (>= 1.8.0, < 2.0)
+      ruby-progressbar (~> 1.7)
+      unicode-display_width (>= 1.4.0, < 3.0)
+    rubocop-ast (1.8.0)
+      parser (>= 3.0.1.1)
+    ruby-progressbar (1.11.0)
     spring (2.1.1)
     sprockets (4.0.2)
       concurrent-ruby (~> 1.0)
@@ -138,6 +157,7 @@ GEM
     thor (1.1.0)
     tzinfo (2.0.4)
       concurrent-ruby (~> 1.0)
+    unicode-display_width (2.0.0)
     websocket-driver (0.7.5)
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.5)
@@ -153,11 +173,12 @@ DEPENDENCIES
   mysql2 (~> 0.5)
   puma (~> 5.0)
   rails (~> 6.1.4)
+  rubocop
   spring
   tzinfo-data
 
 RUBY VERSION
-   ruby 2.7.3p183
+   ruby 2.6.7p197
 
 BUNDLED WITH
-   2.1.4
+   2.2.23