Upgrade proctoring

Impact is that for proctoring backends which support in-exam keepalive
ping messages to ensure the monitoring software is running while
learners have access to proctored exams, we will no longer send ping
requests while the learner is on the ready_to_submit page. These pings
have caused issues for some of our learners in the past, in that some
learners idle on this page waiting for a slow-to-shut-down
application. Pinging while the app is shutting down may've caused
spurious false positives for suspicious learner behavior, and we lose
little integrity by enabling the learner to continue to see the
non-exam-content interstitial, so we're relaxing this rule.

JIRA:CR-1597

requirements/edx/base.txt

@@ -111,7 +111,7 @@ edx-oauth2-provider==1.3.1
 edx-opaque-keys[django]==2.0.1
 edx-organizations==3.0.0
 edx-proctoring-proctortrack==1.0.5
-edx-proctoring==2.2.7
+edx-proctoring==2.2.8
 edx-rbac==1.1.0           # via edx-enterprise
 edx-rest-api-client==3.0.2
 edx-search==1.2.2

requirements/edx/development.txt

@@ -124,7 +124,7 @@ edx-oauth2-provider==1.3.1
 edx-opaque-keys[django]==2.0.1
 edx-organizations==3.0.0
 edx-proctoring-proctortrack==1.0.5
-edx-proctoring==2.2.7
+edx-proctoring==2.2.8
 edx-rbac==1.1.0
 edx-rest-api-client==3.0.2
 edx-search==1.2.2

requirements/edx/testing.txt

@@ -120,7 +120,7 @@ edx-oauth2-provider==1.3.1
 edx-opaque-keys[django]==2.0.1
 edx-organizations==3.0.0
 edx-proctoring-proctortrack==1.0.5
-edx-proctoring==2.2.7
+edx-proctoring==2.2.8
 edx-rbac==1.1.0
 edx-rest-api-client==3.0.2
 edx-search==1.2.2