Fix phutil_console_prompt() under Ubuntu

Summary:
In Ubuntu, sh is dash. It doesn't support read -e, but when we run echo $BASH 2>/dev/null it returns "\n", which is true, so we try to run read -e anyway.

Instead, do an explicit test for the availability of bash, and use it if it's available.

Test Plan: Ran scripts/test/prompt.php --history /tmp/h on Ubuntu and got readline instead of an exception about read -e being illegal.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1302

Differential Revision: https://secure.phabricator.com/D2879