In the cis- isomers, the two bonds between any tw atoms are on the same side. So it adds up the to the net dipole moment, which contributes to stronger London dispersion forces. However, in case of trans- isomers, the two bonds in question are on opposite sides of the plane, and so they cancel each others dipole moments. So in this case, the London dispersion forces are less as the contribution to it by the dipole moment is zero.  

So due to this reason, boiling points are higher for cis- isomers than trans- isomers.