Explicitly create DataFrame

[jeetsukumaran]

The example in the README would fail with:

ERROR: LoadError: MethodError: Cannot `convert` an object of type Matrix{Int64} to an object of type DataFrames.DataFrame

Closest candidates are:
  convert(::Type{DataFrames.DataFrame}, ::DataFrames.SubDataFrame)
   @ DataFrames ~/.julia/packages/DataFrames/58MUJ/src/subdataframe/subdataframe.jl:319
  convert(::Type{T}, ::T) where T
   @ Base Base.jl:84
  DataFrames.DataFrame(::Matrix)
   @ DataFrames ~/.julia/packages/DataFrames/58MUJ/src/dataframe/dataframe.jl:400
  ...

at ``Gillespie.ssa_data()`` .