Fix security issue pointed out by GH (#173)

Author: robertodr

Pipfile

@@ -15,10 +15,14 @@ pyparsing = "*"
 Pygments = "*"
 breathe = "*"
 matplotlib = "*"
-PyYAML = "*"
+pyyaml = ">=4.2b1"
 recommonmark = "*"
 Sphinx = "*"
 sphinx_rtd_theme = "*"
 sphinxcontrib-bibtex = "*"
 yapf = "*"
 fprettify = "*"
+
+[pipenv]
+
+allow_prereleases = true