Modified install.rst to have instructtion for installing using virtualenv as well.
Modified install.rst to have instructtion for installing using virtualenv as
well.

ankurankan/pgmpy,kris-singh/pgmpy,pratyakshs/pgmpy,kislayabhi/pgmpy,kislayabhi/pgmpy,pgmpy/pgmpy,anaviltripathi/pgmpy,sandeepkrjha/pgmpy,anaviltripathi/pgmpy,palashahuja/pgmpy,khalibartan/pgmpy,jhonatanoliveira/pgmpy,abinashpanda/pgmpy,liquidmetal/pgmpy,yashu-seth/pgmpy,pgmpy/pgmpy,yashu-seth/pgmpy,ankurankan/pgmpy,kris-singh/pgmpy,vivek425ster/pgmpy,pratyakshs/pgmpy,sandeepkrjha/pgmpy,liquidmetal/pgmpy,khalibartan/pgmpy,abinashpanda/pgmpy,jhonatanoliveira/pgmpy,palashahuja/pgmpy,vivek425ster/pgmpy