Unpin Sphinx version

docs/supply-chain-build-docs

@@ -37,6 +37,7 @@ RUN echo "deb http://repo.sawtooth.me/ubuntu/ci bionic universe" >> /etc/apt/sou
  && apt-get install -y -q \
     build-essential \
     git \
+    latexmk \
     pep8 \
     python3-colorlog \
     python3-dev \
@@ -75,7 +76,7 @@ RUN apt-get update && apt-get install -y -q \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/* \
     && pip3 install \
-    sphinx==1.5.6 \
+    sphinx \
     sphinxcontrib-httpdomain \
     sphinxcontrib-openapi \
     sphinx_rtd_theme