Add Coverity scan

Coverity scan will run as separate job in second stage. Ii will terminate
itself as soon as possible if build is not triggered by cron. Therefore
it requires to have cron job setup in settings of diffblue/cbmc and
environment variable `COVERITY_SCAN_TOKEN` in the same project settings.

.travis.yml

@@ -127,6 +127,43 @@ jobs:
         - cmake --build build -- -j4
       script: (cd build; ctest -V -L CORE)
 
+    # Run Coverity
+    - stage: Test different OS/CXX/Flags
+      os: linux
+      sudo: required
+      compiler: gcc
+      cache: ccache
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - libwww-perl
+            - g++-5
+        coverity_scan:
+          project:
+            name: "diffblue/cbmc"
+            description: "Travis build of ${TRAVIS_COMMIT}"
+          notification_email: "coverity-scan@diffblue.com"
+          build_command_prepend: "make -C src minisat2-download"
+          build_command: "make -C src -j2"
+          branch_pattern: "develop"
+      before_install:
+        - |
+          if [[ "${TRAVIS_EVENT_TYPE}" != "cron" ]]
+          then
+            echo "This is not a cron build and build is not needed."
+            travis_terminate 0
+          fi
+        - mkdir bin ; ln -s /usr/bin/gcc-5 bin/gcc
+        - sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-5 90
+        - g++ --version
+      # Coverity runs as part of before_script
+      env:
+        - NAME="COVERITY_SCAN"
+        - COMPILER="ccache g++"
+      script: echo "This is coverity build. No need for tests."
+
   allow_failures:
     - <<: *linter-stage
 

README.md

@@ -1,4 +1,5 @@
 [![Build Status][travis_img]][travis] [![Build Status][appveyor_img]][appveyor]
+[![Build Status][coverity_img]][coverity]
 
 [CProver Wiki](http://www.cprover.org/wiki)
 
@@ -48,3 +49,5 @@ License
 [travis_img]: https://travis-ci.org/diffblue/cbmc.svg?branch=master
 [appveyor]: https://ci.appveyor.com/project/diffblue/cbmc/
 [appveyor_img]: https://ci.appveyor.com/api/projects/status/github/diffblue/cbmc?svg=true&branch=master
+[coverity]: https://scan.coverity.com/projects/diffblue-cbmc
+[coverity_img]: https://scan.coverity.com/projects/13552/badge.svg