envsetup: add quotes around PROMPT_COMMAND

Better compatibility for zsh.

Change-Id: Ie010c30580e67e3b5357e3cc869114a525762677
Merge "envsetup: add quotes around PROMPT_COMMAND" into cm-10.1
Merge branch 'cm-10.1' of git://github.com/CyanogenMod/android_build into myjb-next