root@bender:~# adb shell
shell@android:/ $ pm get-install-location
0[auto]
shell@android:/ $ pm set-install-location 2
shell@android:/ $ pm get-install-location
2[external]
shell@android:/ $ logout
root@bender:~#
as you can see from pm help page:
pm get-install-location: returns the current install location.
0 [auto]: Let system decide the best location
1 [internal]: Install on internal device storage
2 [external]: Install on external media
You need to set this value to 2