Provided support for 1.8.6

Tested with 1.8.6 (+ activerecord 2.3.4 and activesupport 2.3.4), 1.8.7 (+ [activerecord 2.3.5 + activesupport 2.3.5] && [activerecord 3.2.11 + activerecord 3.2.11 + activemodel 3.2.11]), and 1.9.3 (+activerecord 3.2.11 + activerecord 3.2.11 + activemodel 3.2.11). 

Ran tests on all versions (on ruby 1.8.6 and 1.8.7 used shoulda-2.10 and mocha 0.9.5, and shoulda 3.3.2 and mocha 0.10.5 on ruby 1.9.3).