fix SSL issue with coverity

.travis.yml

@@ -43,6 +43,7 @@ matrix:
           build_command_prepend: "make clean ; sudo cp $(which g++-5) $(which g++)"
           build_command: "make"
           branch_pattern: coverity_scan
+          before_install: echo -n | openssl s_client -connect scan.coverity.com:443 | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' | sudo tee -a /etc/ssl/certs/ca-certificates.crt
       env: COMPILER=g++-5
 
     # from https://github.com/travis-ci/travis-ci/issues/6120