browsertests: bump phantomjs to 1.9.7

The Ubuntu package is 1.9.0 and there is no effort in Debian to have it
bumped to a more recent version.

Adjust the macro to fetch 1.9.7 (but <1.9.8) using npm AFTER the git
cleanup.

That fix the ZeroBanner job which I have already updated.

Change-Id: I2a5263093b9764a8aecdf63cc797628de8ad8c90