travis ci: switch to python 3.6 and fix gym version

.travis.yml

@@ -10,8 +10,9 @@ script:
                 Conda.add("numpy");
                 Conda.add("scipy==0.17.1");
                 ENV["PYTHONHOME"] = "";
+                Conda.add("python=3.6");
                 Pkg.build("PyCall");'
-    - ~/.julia/conda/3/bin/pip install 'gym[atari]'
+    - ~/.julia/conda/3/bin/pip install 'gym[atari]==0.11.0'
     - julia -e 'using Pkg;
                 Pkg.activate(".");
                 Pkg.test(coverage = true);'

README.md

@@ -29,9 +29,11 @@ end
 then install gym from the command line:
 
 ```
-~/.julia/conda/3/bin/pip install 'gym[all]'
+~/.julia/conda/3/bin/pip install 'gym[all]==0.11.0'
 ```
 
+We only test with gym v0.11.0 at this moment.
+
 ### Install OpenAIGym.jl
 
 ```julia