.travis: Fix installation of non-stock cppzmq

In 86174ee (Travis Tests: Avoid code duplication, 2019-06-05) we added
support for using the environment variable NON_STOCK_CPPZMQ to say if we
use the stock version or the non-stock version.

That commit got the logic wrong. Flip the naming to have now one less
negation.

STOCK_CPPZMQ=ON means that the system cppzmq is used, =OFF that we use
the latest cppzmq from the repo.

Author: t-b

.travis.yml

@@ -4,11 +4,11 @@ language: cpp
 
 env:
   matrix:
-    - OS_TYPE=debian10 CMAKE_BUILD_TYPE=Release SONAR_SCANNER=OFF COVERALLS=OFF NON_STOCK_CPPZMQ=ON
-    - OS_TYPE=debian10 CMAKE_BUILD_TYPE=Debug   SONAR_SCANNER=OFF COVERALLS=OFF NON_STOCK_CPPZMQ=ON
-    - OS_TYPE=debian9  CMAKE_BUILD_TYPE=Debug   SONAR_SCANNER=OFF COVERALLS=OFF NON_STOCK_CPPZMQ=ON
-    - OS_TYPE=debian8  CMAKE_BUILD_TYPE=Debug   SONAR_SCANNER=ON  COVERALLS=ON  NON_STOCK_CPPZMQ=OFF
-    - OS_TYPE=debian7  CMAKE_BUILD_TYPE=Debug   SONAR_SCANNER=OFF COVERALLS=OFF NON_STOCK_CPPZMQ=ON
+    - OS_TYPE=debian10 CMAKE_BUILD_TYPE=Release SONAR_SCANNER=OFF COVERALLS=OFF STOCK_CPPZMQ=ON
+    - OS_TYPE=debian10 CMAKE_BUILD_TYPE=Debug   SONAR_SCANNER=OFF COVERALLS=OFF STOCK_CPPZMQ=ON
+    - OS_TYPE=debian9  CMAKE_BUILD_TYPE=Debug   SONAR_SCANNER=OFF COVERALLS=OFF STOCK_CPPZMQ=ON
+    - OS_TYPE=debian8  CMAKE_BUILD_TYPE=Debug   SONAR_SCANNER=ON  COVERALLS=ON  STOCK_CPPZMQ=OFF
+    - OS_TYPE=debian7  CMAKE_BUILD_TYPE=Debug   SONAR_SCANNER=OFF COVERALLS=OFF STOCK_CPPZMQ=ON
 
 notifications:
   email: false
@@ -44,7 +44,7 @@ before_script:
   - docker build --build-arg APP_UID=$(id -u) --build-arg APP_GID=$(id -g) -t cpp_tango .travis/${OS_TYPE}
   - docker run --name cpp_tango -e TANGO_HOST=${TANGO_HOST} -e BINTRAY_USER_NAME=tango-ci -e BINTRAY_API_KEY=${CI_BINTRAY_API_KEY} -e COVERALLS_REPO_TOKEN=${COVERALLS_REPO_TOKEN} --link tango_cs:tango_cs -v `pwd`:/home/tango/src -v `pwd`/idl:/home/tango/idl -v `pwd`/cppzmq:/home/tango/cppzmq -v `pwd`/coveralls-cmake:/home/tango/coveralls-cmake -v `pwd`/build-wrapper-linux-x86:/home/tango/build-wrapper-linux-x86 -dit cpp_tango
   - .travis/install_tango_idl.sh
-  - (test ${NON_STOCK_CPPZMQ} = "ON" && .travis/install_cppzmq.sh) || true
+  - (test ${STOCK_CPPZMQ} = "OFF" && .travis/install_cppzmq.sh) || true
 #work around gcov ignored by sonar
   - sudo mkdir /home/tango && sudo mkdir /home/tango/src && sudo mount --bind `pwd` /home/tango/src