Run the Rubocop CI check only when a Ruby file is changed

[lslezak]

Problem

• The Rubocop check is run always, even when no Ruby file is modified
• This makes CI slower and wastes some server resources

Solution

• Put the Rubocop job into a separate file and use the path specifiers so it is run only when a matching file is updated

[coveralls]

coverage: 72.134%. remained the same when pulling 766f3dd on ci-rubocop into 4df4c55 on master.