NO-JIRA Workaround Travis CI package installation issues (#311)

diff --git a/.travis.yml b/.travis.yml
index b49b97e..2988e0a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -58,7 +58,9 @@
     os: linux
     dist: focal
     before_install:
-    - sudo apt-get install -y gcc-10 g++-10
+      # https://travis-ci.community/t/clang-10-was-recently-broken-on-linux-unmet-dependencies-for-clang-10-clang-tidy-10-valgrind/11527
+      - sudo apt-get install -yq --allow-downgrades libc6=2.31-0ubuntu9.2 libc6-dev=2.31-0ubuntu9.2
+      - sudo apt-get install -y gcc-10 g++-10
     env:
     - CC=gcc-10
     - CXX=g++-10
@@ -70,6 +72,8 @@
     os: linux
     dist: focal
     before_install:
+    # https://travis-ci.community/t/clang-10-was-recently-broken-on-linux-unmet-dependencies-for-clang-10-clang-tidy-10-valgrind/11527
+    - sudo apt-get install -yq --allow-downgrades libc6=2.31-0ubuntu9.2 libc6-dev=2.31-0ubuntu9.2
     - sudo apt-get install -y clang-11 llvm-11-dev
     env:
     - CC=clang-11
@@ -134,7 +138,6 @@
     - sasl2-bin
     - swig
     - python-dev
-    - valgrind
     - ruby
     - ruby-dev
     - python3-dev
@@ -151,11 +154,15 @@
 # https://travis-ci.community/t/macos-build-fails-because-of-homebrew-bundle-unknown-command/7296/10
     update: true
 
+before_install:
+  # https://travis-ci.community/t/clang-10-was-recently-broken-on-linux-unmet-dependencies-for-clang-10-clang-tidy-10-valgrind/11527
+  - if [[ "${TRAVIS_DIST}" == "focal" ]]; then sudo apt-get install -yq --allow-downgrades libc6=2.31-0ubuntu9.2 libc6-dev=2.31-0ubuntu9.2; fi
+  - if [[ "${TRAVIS_DIST}" == "focal" ]]; then sudo -E apt-get -yq --no-install-suggests --no-install-recommends --allow-downgrades --allow-remove-essential --allow-change-held-packages install valgrind -o Debug::pkgProblemResolver=yes; fi
+
 install:
 - echo "Using PYTHON=${PYTHON:=python}"
-# use older version of virtualenv to workaround https://github.com/pypa/virtualenv/issues/1873
 - ${PYTHON} -m pip install --user --upgrade pip
-- ${PYTHON} -m pip install --user coverage setuptools wheel tox virtualenv==20.0.23
+- ${PYTHON} -m pip install --user coverage setuptools wheel tox virtualenv
 # PROTON-2125 suppress annoying deprecation warning from Minitest in Ruby tests
 - gem install minitest:4.3.2 simplecov:0.18.0 codecov