Fix method overwrite warning when loading the package (#255)

This was caused by providing a default value of `nothing` for the
`model` argument in the `Problem` constructor, which ends up creating a
two-argument method that is also created further down by virtue of using
default values.

By simply removing the default value for `model`, we remain at feature
parity with the methods prior to the change and we don't overwrite
anything.

src/problems.jl

@@ -29,7 +29,7 @@ mutable struct Problem
     solution::Solution
 
     function Problem(head::Symbol, objective::AbstractExpr,
-                     model::Union{MathProgBase.AbstractConicModel, Nothing}=nothing,
+                     model::Union{MathProgBase.AbstractConicModel, Nothing},
                      constraints::Array=Constraint[])
         if sign(objective)== Convex.ComplexSign()
             error("Objective can not be a complex expression")