Remove statsmodels, scikit-learn, and other dependencies; add gross leverage to performance stats summary

[gusgordon]

Removing the statsmodels dependency was straightforward, as the calc_multifactor function was unused which was the only function that needed statsmodels. Removing scikit-learn required replacing the scale function we were dependent on with an equivalent, which is now located in utils.py. Remove jsonschema, funcsigs, mock, pyparsing, and python-dateutil dependencies, and remove an unnecessary line from travis.

Also add mean gross leverage in the performance stats summary as well as the bootstrapped version.