Edit
As stated by Tyler Rinker, one way is to use the function factor
to order factor levels (in this case, months).
I created some data to use with ggplot, but you can adapt to your data and use the logic with ggmap.
library(ggplot)
x = c(6.2, 2.3, 0, 1.54, 2.17, 6.11, 0.3,
1.39, 5.14, 12.52, 12.57, 7.13, 13.71)
y = c(7.89, 7.63, 5.29, 8.38, 8.37, 10.5, 21.5,
16.65, 23.76, 1.77, 1.8, 10.49, 14.01)
month = month.abb # system constant in correct sort order.
mypoints = data.frame(cbind(x,y,month))
mypoints$month = factor(mypoints$month,
levels=month.abb )
ggplot(data = mypoints,aes(x,y)) +
geom_point(aes(color=month), alpha=0.5, size=5)