fix user manual references

[yizhang-yiz]

• Add \stanversion and \torstenversion for future version update in the manual.
• Remove hanging \cite{mixedsolver} that was causing bib failure
• Fix caption for `univariate_integral1 example figure.

[charlesm93]

I'm close to submitting the mixed solver manuscript for publication. In the meantime, a good reference would be the poster Bill and I presented at ACoP8, see http://metrumrg.com/wp-content/uploads/2017/10/Gillespie_MixedSolver_ACOP2017.pdf.

Here's the code for the .bib file:

@conference{Margossian&Gillespie:2017b,
    Author = {Margossian, Charles C and Gillespie, William R},
    Booktitle = {Journal of Pharmacokinetics and Pharmacodynamics},
    Month = {October},
    Title = {Gaining Efficiency by Combining Analytical and Numerical Methods to Solve ODEs: Implementation in Stan and Application to Bayesian PK/PD},
    Volume = {47},
    Year = {2017}}