added requirement for julia 0.3- for Datetime v0.1.5 and v0.1.6

Datetime versions 0.1.5 and 0.1.6 doesn't include the requirement for julia 0.3- which is part of the requirements for Datetime version 0.1.4.  This change adds this requirement.  The REQUIRE file should also be updated on @quinnj's Datetime.jl project.