fix test failure due to upstream rubocop changes

bbatsov/rubocop@ff167d8f changed Rubocop -> RuboCop
fixup ruby style in test suite

upstream rubocop now recommends to use backticks instead of %x
rspec/mocha now requires symbol for mock_with

Resolves error:

    Unknown mocking framework: "mocha".
    Pass a module or one of [:rspec, :flexmock, :rr, :mocha, :nothing]