rollback LsqFit to 0.13

Author: eford

Project.toml

@@ -48,7 +48,7 @@ FileIO = "1.4"
 Interpolations = "0.12, 0.13, 0.14, 0.15"
 JLD2 = "0.2, 0.3, 0.4"
 KernelFunctions = "0.9.7, 0.10"
-LsqFit = "0.11, 0.12, 0.13, 0.15"
+LsqFit = "0.11, 0.12, 0.13" # replace estimate_covar with vcov starting in 0.14 
 MultivariateStats = "0.7, 0.8, 0.9, 0.10"
 NaNMath = "0.3, 1"
 Optim = "1"