travis: Specify pytest-rerunfailures==9.1 to avoid regression

Seems the recently released version 9.1.1 has regressed, and isn't actually
rerunning failed tests. Pinning it to 9.1 seems to work however.

.travis/build.sh

@@ -54,7 +54,7 @@ if [ "$NO_PYTHON" != 1 ]; then
     pip3 install --user -U --quiet --progress-bar off \
 	 blinker \
 	 pytest-sentry \
-	 pytest-rerunfailures
+	 pytest-rerunfailures==9.1
 fi
 
 echo "Configuration which is going to be built:"

contrib/pyln-testing/requirements.txt

@@ -1,4 +1,5 @@
 pytest==5.*
+pytest-rerunfailures==9.1
 Flask==1.1.1
 cheroot==8.2.1
 ephemeral-port-reserve==1.1.1