vmtests: ensure rst2man is installed, needed for bpftool selftests

Ensure rst2man package is installed. This is now a dependency for
selftests/bpf.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>

travis-ci/vmtest/run_vmtest.sh

@@ -19,6 +19,7 @@ wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
 echo "deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic main" | sudo tee -a /etc/apt/sources.list
 sudo apt-get update
 sudo apt-get -y install clang-12 lld-12 llvm-12
+sudo apt-get -y install python-docutils # for rst2man
 
 travis_fold end install_clang