Use 'double' instead of 'mock' for rspec-mocks

'mock' is deprecated after rspec-mocks 2.14.0.rc1(2013-05-27 released).
'mock' method may be removed for rspec-mocks 3.0.0.

And, this fix can remove following warnings.
DEPRECATION: mock is deprecated. Use double instead. Called from spec/coveralls/simplecov_spec.rb:64:in `block (4 levels) in <top (required)>'.