Do not explicitly install conan

Instead, let it be installed as a dependency of `conan-package-tools`
to avoid trouble with the fact that pip is really bad at version
resolution, and that up-to-date version of the `conan` package is not
supported by up-to-date version of the `conan-package-tools` package.

.travis.yml

@@ -282,7 +282,7 @@ matrix:
         - "3.7"
       dist: xenial
       install:
-        - pip install conan conan-package-tools
+        - pip install conan-package-tools
       env:
         - CONAN_GCC_VERSIONS=8
         - CONAN_DOCKER_IMAGE=conanio/gcc8