Fixed a bug which only occurs in the few months before February

This is the fundamental problem:

    2.1.2 :005 > Time.zone.now + 3.months
    => Sat, 28 Feb 2015 03:17:24 UTC +00:00
    2.1.2 :006 > Time.zone.now + 3.months + 3.months
    => Thu, 28 May 2015 03:17:32 UTC +00:00
    2.1.2 :007 > Time.zone.now + 6.months
    => Sat, 30 May 2015 03:17:37 UTC +00:00

I changed the test to use the "3.months + 3.months" formulation and it
now passes.