Upgrade Rubocop to 0.28.0 for Travis builds

Change-Id: I8694b2f3aebe0fcfa52d27fc97d2eb04a7661186
Reviewed-on: http://gerrit.causes.com/45333
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>

Author: sds

Gemfile

@@ -1,6 +1,6 @@
 source 'https://rubygems.org'
 
 # Pin Rubocop for Travis builds.
-gem 'rubocop', '0.27.1'
+gem 'rubocop', '0.28.0'
 
 gemspec

lib/overcommit/git_repo.rb

@@ -23,7 +23,6 @@ def extract_modified_lines(file_path, options)
 
       `git diff --no-ext-diff -U0 #{flags} -- #{file_path}`.
         scan(DIFF_HUNK_REGEX) do |start_line, lines_added|
-
         lines_added = (lines_added || 1).to_i # When blank, one line was added
         cur_line = start_line.to_i