virtualenvwrapper.sh: always export VIRTUALENVWRAPPER_HOOK_DIR

Previously, if VIRTUALENVWRAPPER_HOOK_DIR is set to a nonempty value but not
exported, virtualenvwrapper_initialize would assume it is available in the
environment, and that leads to confusing problems like #276 where
user_scripts.make_hook attempts to create hooks like
'/Users/jacob/venvs/$VIRTUALENVWRAPPER_HOOK_DIR/premkproject'.

In this commit, we make sure VIRTUALENVWRAPPER_HOOK_DIR is always exported, so
that $VIRTUALENVWRAPPER_HOOK_DIR is always expanded by user_scripts.get_path.

Author: zmwangx

virtualenvwrapper.sh

@@ -304,8 +304,9 @@ function virtualenvwrapper_initialize {
     # Set the location of the hook scripts
     if [ "$VIRTUALENVWRAPPER_HOOK_DIR" = "" ]
     then
-        export VIRTUALENVWRAPPER_HOOK_DIR="$WORKON_HOME"
+        VIRTUALENVWRAPPER_HOOK_DIR="$WORKON_HOME"
     fi
+    export VIRTUALENVWRAPPER_HOOK_DIR
 
     mkdir -p "$VIRTUALENVWRAPPER_HOOK_DIR"