Indicate scripts/bootstrap.py should be run by python2.

TBR=jmadill@chromium.org
BUG=angleproject:2209

Change-Id: I2ccfee1407cd4ce457aa28a374ac202b8281d40b
Reviewed-on: https://chromium-review.googlesource.com/862618Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>