Running in parallel can cause random errors with Devel::Cover

see https://travis-ci.org/ufal/perl-pmltq-server/jobs/65417093
and probably related to pjcj/Devel--Cover#73

.travis.yml

@@ -5,7 +5,7 @@ perl:
   - "5.16"
   - "5.14"
 env:
-  - "HARNESS_OPTIONS=j9 MOJO_MAIL_TEST=1 PMLTQ_SERVER_TESTDB='mongodb://127.0.0.1:27017/pmltq-server-test'"
+  - "MOJO_MAIL_TEST=1 PMLTQ_SERVER_TESTDB='mongodb://127.0.0.1:27017/pmltq-server-test'"
 before_install:
   - "sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10"
   - "echo 'deb http://repo.mongodb.org/apt/ubuntu '$(lsb_release -sc)'/mongodb-org/3.0 multiverse' | sudo tee /etc/apt/sources.list.d/mongodb-org-3.0.list"