replace rep with repeat

I guess ```rep``` must have previously existed and no longer does? Or there's an unstated dependency? Anyway, I believe my change has the intended behavior.

I ran into this running Example 2 from the README, and until this change I got this error:

ERROR: rep not defined
 in run_seqmc at /Users/david/.julia/MCMC/src/runners/SeqMC.jl:104
 in run at /Users/david/.julia/MCMC/src/runners/runners.jl:28