.travis.yml: Clone cppzmq only when required

Author: t-b

.travis.yml

@@ -32,7 +32,7 @@ before_install:
   - docker pull tangocs/mysql:9.2.2
   - docker pull tangocs/tango-cs:latest
   - git clone https://github.com/JoakimSoderberg/coveralls-cmake.git
-  - git clone -b v4.2.2 https://${CI_USER_TOKEN}@github.com/zeromq/cppzmq.git cppzmq
+  - (test ${STOCK_CPPZMQ} = "OFF" && git clone -b v4.2.2 https://${CI_USER_TOKEN}@github.com/zeromq/cppzmq.git cppzmq) || mkdir cppzmq
   - git clone -b tango-9-lts https://${CI_USER_TOKEN}@github.com/tango-controls/tango-idl.git idl
   - wget https://sonarcloud.io/static/cpp/build-wrapper-linux-x86.zip && unzip build-wrapper-linux-x86.zip