Remove rubocop because it causes backtrace

MSP-11046

See https://gist.github.com/limhoff-r7/7c398b5f4c44ed40cf1f

Gemfile

@@ -15,8 +15,6 @@ group :db do
 end
 
 group :development do
-  # Style/sanity checking Ruby code
-  gem 'rubocop'
   # Markdown formatting for yard
   gem 'redcarpet'
   # generating documentation

Gemfile.lock

@@ -45,7 +45,6 @@ GEM
     arel (3.0.3)
     arel-helpers (2.0.1)
       activerecord (>= 3.1.0, < 5)
-    ast (2.0.0)
     bcrypt (3.1.7)
     builder (3.0.4)
     coderay (1.1.0)
@@ -88,12 +87,8 @@ GEM
     nokogiri (1.6.3.1)
       mini_portile (= 0.6.0)
     packetfu (1.1.9)
-    parser (2.1.9)
-      ast (>= 1.1, < 3.0)
-      slop (~> 3.4, >= 3.4.5)
     pcaprub (0.11.3)
     pg (0.17.1)
-    powerpack (0.0.9)
     pry (0.10.0)
       coderay (~> 1.1.0)
       method_source (~> 0.8.1)
@@ -112,7 +107,6 @@ GEM
       rake (>= 0.8.7)
       rdoc (~> 3.4)
       thor (>= 0.14.6, < 2.0)
-    rainbow (2.0.0)
     rake (10.3.2)
     rdoc (3.12.2)
       json (~> 1.4)
@@ -138,14 +132,6 @@ GEM
       rspec-core (~> 2.99.0)
       rspec-expectations (~> 2.99.0)
       rspec-mocks (~> 2.99.0)
-    rubocop (0.23.0)
-      json (>= 1.7.7, < 2)
-      parser (~> 2.1.9)
-      powerpack (~> 0.0.6)
-      rainbow (>= 1.99.1, < 3.0)
-      ruby-progressbar (~> 1.4)
-    ruby-progressbar (1.5.1)
-      activesupport (>= 3.0.0)
     rubyntlm (0.4.0)
     rubyzip (1.1.6)
     shoulda-matchers (2.6.2)
@@ -185,7 +171,6 @@ DEPENDENCIES
   redcarpet
   rspec (>= 2.12, < 3.0.0)
   rspec-rails (>= 2.12, < 3.0.0)
-  rubocop
   shoulda-matchers
   simplecov (= 0.5.4)
   timecop