travis-ci: Add configuration for Coverity Scan

Run Coverity Scan builds on Travis CI, but only when pushing into the
"coverity" branch.

.travis.yml

@@ -12,7 +12,7 @@ before_script:
     - ./configure
 
 script:
-    - make
+    - if [ "$COVERITY_SCAN_BRANCH" != 1 ]; then make; fi
 
 compiler:
     - clang
@@ -26,3 +26,25 @@ notifications:
     on_failure: always
     skip_join: true
 #  email: false
+
+# Don't use Clang for Coverity Scan.
+matrix:
+  exclude:
+    - compiler: clang
+    - env: COVERITY_SCAN_BRANCH=1
+
+env:
+  global:
+    # This is the encrypted COVERITY_SCAN_TOKEN, created via the "travis
+    # encrypt" command using the project repository's public key.
+    - secure: "ggJ9c/VfKcwtrwz/My+ne4My7D8g3qi3vz5Hh+yLiri0+oIXCy313ZD6ssIEY/5beQZEOnuHhBgBJd/Y3clSQNc2M9fRNc+wxOkIO992lgnY0MZJN3y9MLfpqUbTClhU9Fst0qXQqGpI6UI8yz1tj7yKi7DPrycJLRrjMpyTfyo="
+
+addons:
+  coverity_scan:
+    project:
+      name: "monitoring-plugins/monitoring-plugins"
+      description: "Monitoring Plugins"
+    notification_email: team@monitoring-plugins.org
+    build_command_prepend: tools/setup && ./configure
+    build_command: make
+    branch_pattern: coverity