Fixed warnings when run tests with ruby -w

Here is the messages:

.../lib/coveralls.rb:66: warning: `&' interpreted as argument prefix
.../lib/coveralls/api.rb:20: warning: (...) interpreted as grouped expression
.../lib/coveralls/api.rb:22: warning: (...) interpreted as grouped expression
.../lib/coveralls/api.rb:25: warning: (...) interpreted as grouped expression
.../lib/coveralls/api.rb:27: warning: (...) interpreted as grouped expression
.../lib/coveralls.rb:77: warning: instance variable @testing not initialized
.../lib/coveralls.rb:90: warning: instance variable @noisy not initialized