install ipython for current python version 2.x

Use ipython2 to start a ipython2 shell, if you need to install for python2 use pip2 install ipython. pip obviously points to python3 on your system so specifying pip2 will install ipython for python2.

Whatever the shebang points to will mean typing just ipython will start a shell for that version of python so if you had #!/usr/bin/python3.4 then ipython will start an ipython3 shell and vice-versa. Unless you have Ipython actually installed for python2 then changing the shebang won't do anything but error.