support for running mini tests in RubyMine

add `gem 'minitest-reporters', require: false` to your `bundle.d/local.rb`
and set ENV['RUBYMINE'] in Run/Debug configuration defaults
move ruby-linter to minitests
add source code test for not having `rescue Exception`
test ENV variables not to be used
add test for soft-tabs and extend scope of trailing space test

fix current violations
add gettext string test
fix soft tab test
Merge pull request #1993 from pitr-ch/quick-fix/ruby-lint

move ruby-linter to a minitest spec