Roll back libjs version to 1.7.0

Roll back libjs version to 1.7.0

The install version of libjs (Spidermonkey) had been bumped to the
latest, 1.8.5, but it turned out that the tests for that had probably
been run on a system that had an old version of libjs source and/or
installed headers/libraries in place, as building libjs 1.8.5 is a
different beast entirely (different directory structure, etc).

In addition, there are several bugs along the lines of
https://rt.cpan.org/Public/Bug/Display.html?id=75058 that strongly
suggest that libjs-1.8.5 is not compatible with the
JavaScript::SpiderMonkey 0.21 Perl module.

Therefore, roll back to the tried-and-true libjs 1.7.0.

Signed-off-by: Dan Scott <dscott@laurentian.ca>