CI: travis-ci: help Coverity to recognize abort()

generally haproxy uses (*(volatile int*)1=0) for abort. It is not recognized
by static analyzers, e.g. Coverity scan as abort, so fallback to abort() was
introduced in previous commit for code analysis purpose. Let us explicitely
use it for Coverity build job
diff --git a/.travis.yml b/.travis.yml
index ca867d9..8850850 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -55,7 +55,7 @@
   - os: linux
     if: type == cron
     compiler: clang
-    env: TARGET=linux-glibc COVERITY_SCAN_PROJECT_NAME="Haproxy" COVERITY_SCAN_BRANCH_PATTERN="*" COVERITY_SCAN_NOTIFICATION_EMAIL="chipitsine@gmail.com" COVERITY_SCAN_BUILD_COMMAND="make CC=clang TARGET=$TARGET $FLAGS 51DEGREES_SRC=$FIFTYONEDEGREES_SRC"
+    env: TARGET=linux-glibc COVERITY_SCAN_PROJECT_NAME="Haproxy" COVERITY_SCAN_BRANCH_PATTERN="*" COVERITY_SCAN_NOTIFICATION_EMAIL="chipitsine@gmail.com" COVERITY_SCAN_BUILD_COMMAND="make CC=clang DEFINE=-DDEBUG_USE_ABORT TARGET=$TARGET $FLAGS 51DEGREES_SRC=$FIFTYONEDEGREES_SRC"
     script:
       - |
         if [ ! -z ${COVERITY_SCAN_TOKEN+x} ]; then