There's really no problem in this situation. Lots of people don't know if they'll be over (or under) the limit until the end of the year. You can make the full contribution to the Roth now, and if it, at the end of the year, you're over the limit, you can reverse the contribution without penalty as long as you withdraw (a) the amount of the contribution and
(b) any earnings attributed to that contribution before the tax filing deadline for the year in question (so if the contribution is for 2013 the deadline for penalty-free withdrawal is normally April 15, 2014). Usually all this involves is filling out a form and sending it to the custodian. They compute the earnings attributable and all you do is tell them where to transfer the money. But of course you should check with your IRA custodian to see what their procedure is.
See IRS Publication 590
pp. 65-66 for the official explanation.