avoid prepending system ruby to PATH

System ruby is already on PATH (that's the definition of system ruby) and by
duplicating its path by putting it in front, we can break the user's
environment.

Fixes #275

libexec/rbenv-exec

@@ -7,7 +7,9 @@ if [ "$1" = "--complete" ]; then
   exec rbenv shims --short
 fi
 
+export RBENV_VERSION="$(rbenv-version-name)"
 RBENV_COMMAND="$1"
+
 if [ -z "$RBENV_COMMAND" ]; then
   echo "usage: rbenv exec COMMAND [arg1 arg2...]" >&2
   exit 1
@@ -21,5 +23,7 @@ for script in $(rbenv-hooks exec); do
 done
 
 shift 1
-export PATH="${RBENV_BIN_PATH}:${PATH}"
+if [ "$RBENV_VERSION" != "system" ]; then
+  export PATH="${RBENV_BIN_PATH}:${PATH}"
+fi
 exec -a "$RBENV_COMMAND" "$RBENV_COMMAND_PATH" "$@"