Run latex build both on push and pull request. (#91)

* Run latex build both on push and pull request.

This should allow us to preview latex updates in PDF form before they are merged.

Author: Micah Zoltu

.github/workflows/build_latex.yml

@@ -1,5 +1,9 @@
 name: Build LaTeX document
-on: [push]
+on:
+  push:
+    branches:
+      master
+  pull_request:
 jobs:
   build_latex:
     runs-on: ubuntu-latest