Conan build configuration simplified.

.travis.yml

     - env: COMPILER=clang-7
     - env: COMPILER=clang-6
     - env: COMPILER=clang-5
-    - env: CONAN_GCC_VERSIONS=9 CONAN_DOCKER_IMAGE=conanio/gcc9
+    - env: CONAN_GCC_VERSIONS=9
 
 
 before_install:

script/conan_build.sh

 
 set -ex
 
+export CONAN_DOCKER_IMAGE=conanio/gcc${CONAN_GCC_VERSIONS}
+
 pip install -U conan conan_package_tools
 python build.py