Fix jqXHR use in unit tests run by CI

When running our unit tests on Sauce via Circle, we POST the test
results back to Circle CI with jQuery, but in 4e2a50f on #653
(build.release-process) "New 2-step release process:
script/{prep,push}-release.sh" we downgraded the jQuery our tests use
from 1.7.2 to 1.5.2, apparently before jqXHR::always was added.

I figured this out thanks to Sauce's nifty feature of letting you take
over in-progress JS Unit Test sessions, though it was frustrating
because Developer Tools kept closing immediately after opening, as you
can see in the video of the session:
https://saucelabs.com/beta/tests/cdc99f378ed54305bba17b9d722fa90e/watch
(but if you freeze it at 00:37 you can see the `$.post(...).always is
not a function` that lead to this conclusion)

https://mathquill.slack.com/archives/github/p1481602022000016
diff --git a/test/unit.html b/test/unit.html
index 280a0c9..9369824 100644
--- a/test/unit.html
+++ b/test/unit.html
@@ -77,7 +77,7 @@
 
         runner.on('end', function() {
           setTimeout(function() {
-            $.post(post_xunit_to, xunit).always(function() {
+            $.post(post_xunit_to, xunit).complete(function() {
               window.mochaResults = runner.stats;
               window.mochaResults.reports = failedTests;
             });