Checkout master branch

The git checkout command was in the continuous deployment script
in order to get the correct information to go in the footer of the
PDF. However, since the build is happening on the basis of a tag,
the git repository was in a detached HEAD state, meaning that the
branch information that showed up in the footer of the PDF was
'origin/HEAD', rather than 'master'. To fix this, the script
now runs `git checkout master` instead of `git checkout`.

This solution is a bit hacky because, in principle, the tag that
is being built might be a different commit than the most recent
commit on the master branch. However, it should work well enough
for now, but it would be nice to have a more robust solution in
the future.

circle.yml

@@ -7,7 +7,7 @@ machine:
 checkout:
   post:
     - ./circle-ci-setup/install-git-hooks.sh
-    - git checkout
+    - git checkout master
 
 dependencies:
   pre:
@@ -50,4 +50,4 @@ deployment:
 general:
   branches:
     ignore:
-      - /.*?/
+      - /.*?/