[Gemfile] Pin rubocop to ~> 0.26.0

Rubocop 0.27.0 causes CocoaPods to have many offences

Gemfile

@@ -34,7 +34,7 @@ group :development do
   # Integration tests
   gem 'diffy'
   gem 'clintegracon'
-  gem 'rubocop'
+  gem 'rubocop', '~> 0.26.0'
 end
 
 group :debugging do

Gemfile.lock

@@ -144,9 +144,9 @@ GEM
       ffi (>= 0.5.0)
     rb-kqueue (0.2.3)
       ffi (>= 0.5.0)
-    rubocop (0.27.0)
+    rubocop (0.26.1)
       astrolabe (~> 1.3)
-      parser (>= 2.2.0.pre.6, < 3.0)
+      parser (>= 2.2.0.pre.4, < 3.0)
       powerpack (~> 0.0.6)
       rainbow (>= 1.99.1, < 3.0)
       ruby-progressbar (~> 1.4)
@@ -187,7 +187,7 @@ DEPENDENCIES
   pry
   rake
   rb-fsevent
-  rubocop
+  rubocop (~> 0.26.0)
   ruby-prof
   webmock
   xcodeproj!