Set VIRTUALENVWRAPPER_SCRIPT portably

- `${.sh.file}` was ksh syntax, but ksh support was dropped in commit
  d736549
- Instead of testing only for certain specific shells (and excluding
  all others), use `$BASH_SOURCE` if it's set, otherwise fall back
  to `$0` which should work with more shells (hopefully).

  Fixes #105

Author: ferdnyc

virtualenvwrapper.sh

@@ -96,14 +96,11 @@ export VIRTUALENVWRAPPER_WORKON_CD=${VIRTUALENVWRAPPER_WORKON_CD:-1}
 # Remember where we are running from.
 if [ -z "${VIRTUALENVWRAPPER_SCRIPT:-}" ]
 then
-    if [ -n "$BASH" ]
+    if [ -n "${$BASH_SOURCE:-}" ]
     then
         export VIRTUALENVWRAPPER_SCRIPT="$BASH_SOURCE"
-    elif [ -n "$ZSH_VERSION" ]
-    then
-        export VIRTUALENVWRAPPER_SCRIPT="$0"
     else
-        export VIRTUALENVWRAPPER_SCRIPT="${.sh.file}"
+        export VIRTUALENVWRAPPER_SCRIPT="$0"
     fi
 fi