chore(*): stop testing on Edge 15 due to its instability

Edge 15 disconnects from Karma frequently causing extreme build instability.
We are testing on Edge 16 & Edge 17 should be released soon anyway.

Closes #16516

Author: mgol

scripts/travis/build.sh

@@ -11,12 +11,14 @@ export SAUCE_ACCESS_KEY
 BROWSER_STACK_ACCESS_KEY=$(echo "$BROWSER_STACK_ACCESS_KEY" | rev)
 SAUCE_ACCESS_KEY=$(echo "$SAUCE_ACCESS_KEY" | rev)
 
+# TODO: restore "SL_EDGE-1" once Sauce Labs adds Edge 17 and "SL_EDGE-1" refers
+# to version 16. Edge 15 disconnects from Karma frequently causing extreme build instability.
 BROWSERS="SL_Chrome,SL_Chrome-1,\
 SL_Firefox,SL_Firefox-1,\
 SL_Safari,SL_Safari-1,\
 SL_iOS_10,SL_iOS_11,\
 SL_IE_9,SL_IE_10,SL_IE_11,\
-SL_EDGE,SL_EDGE-1"
+SL_EDGE"
 
 case "$JOB" in
   "ci-checks")