Use an up to date conan package version again

Last time it was fixed to a specific version because the `conan`
and the `conan-package-tools` package that `pip install` would
gather were not compatible, let's hope it won't happen again.

.travis.yml

@@ -276,7 +276,7 @@ matrix:
         - "3.7"
       dist: xenial
       install:
-        - pip install conan==1.10.2 conan-package-tools
+        - pip install conan conan-package-tools
       env:
         - CONAN_GCC_VERSIONS=8
         - CONAN_DOCKER_IMAGE=conanio/gcc8