Enable warning option on rspec task
Declare an ivar via #initialize

Why
----

This commit removes the following interpreter warning.

* warning: instance variable @{ivar} not initialized

Reproduction
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* ruby 2.0.0p195 (2013-05-14 revision 40734)
* ruby 1.9.3p426 (2013-05-14 revision 40737)
* `$VERBOSE = true` on spec
Merge pull request #2 from kachick/improve-sweep_warnings

Remove interpreter warnings