Replace non-portable '\e' with '\033' (ESC)

Summary: Warp complains about the use of '\e' as a character literal, which indeed is not allowed (see e.g. http://en.cppreference.com/w/cpp/language/escape). However, I just checked and gcc does accept the sequence. To keep code portable I replaced \e with \033.

Reviewed By: @jdelong

Differential Revision: D1429142