chore(env): Update pandoc and pandoc-crossref

Use the latest version of pandoc which changes a collection of things
with wider compatibility.

environment.yml

@@ -3,15 +3,15 @@ dependencies:
   - python=3.7
   - pip=19.3
   - setuptools==41.0.1
-  - pandoc==2.7.3
+  - pandoc==2.8.1
   - conda-forge::tectonic==0.1.11
   - malramsay::biber==2.11
   - pre_commit=1.18
   - nodejs==10.13.0
   - conda-forge::graphviz==2.42.3
   - conda-forge::cairosvg==2.4.2
   - conda-forge::pandocfilters==1.4.2
-  - conda-forge::pandoc-crossref==0.3.4.2
+  - conda-forge::pandoc-crossref==0.3.5.0.2
   - pip:
       - hsluv==0.0.2
       - gitlint==0.12.0