Reconfigures manuscript on CircleCI to run only on master

See #211. In a nutshell, the free CircleCI account under which this is
running has a cap on CPU time that resets every month (I believe). This
means that builds that aren't well justified should not be undertaken,
so that CPU time remains when it is well (or at least better) justified.

Since we meanwhile have a more extensive test suite, it is hard to imagine
that between the tests and the vignettes there is anything that these would
miss but that the manuscript (which isn't very thorough in what of the
API it is touching) would catch. Hence, rebuild only on master.

Closes #211.

Author: hlapp

.circleci/config.yml

@@ -7,4 +7,13 @@ jobs:
       - checkout
       - run: R -e "devtools::install(dep=T); rmarkdown::render('manuscripts/manuscript.Rmd')"
 
+workflows:
+  version: 2
+
+  build_only:
+    jobs:
+      - build:
+        filters:
+          branches:
+            only: master