Provide the \tightlist command

Just as it is provided the default.latex template.
Pandoc-1.14 expexts this command.

default.beamer

@@ -107,6 +107,8 @@ $endif$
 \setlength{\parindent}{0pt}
 \setlength{\parskip}{6pt plus 2pt minus 1pt}
 \setlength{\emergencystretch}{3em}  % prevent overfull lines
+\providecommand{\tightlist}{%
+  \setlength{\itemsep}{0pt}\setlength{\parskip}{0pt}}
 $if(numbersections)$
 $else$
 \setcounter{secnumdepth}{0}