Run CircleCI builds every day

Modern ecosystem needs to go fast, die young, break early, and ship
often. Therefore we can never be sure that if a commit was successfully
built today then the same code can be built tomorrow. Transitive
dependencies come and go, new toolchain versions get released, etc.
We do not have every dependency pinned, so we should be testing
regularly to see if anything breaks.

Let's run CircleCI test suite every day at 5:00 to make sure that we see
the scheduled breakage early in the morning, not when some poor soul
submits a pull request and *then* learns that something does not work.

.circleci/config.yml

@@ -465,3 +465,23 @@ workflows:
       - php70
       - php71
       - integration_tests
+  nightly:
+    # Apparently CircleCI does not have a "push" or "pull request" trigger
+    # so we have to have a separate workflow with the same job list.
+    triggers:
+      - schedule:
+          cron: "0 5 * * *"
+          filters:
+            branches:
+              only:
+                - master
+                - stable
+    jobs:
+      - analyze
+      - android
+      - x86_64
+      - jsthemis
+      - php5
+      - php70
+      - php71
+      - integration_tests