Fix TravisCI build

Author: ntninja

.travis.yml

@@ -10,10 +10,8 @@ matrix:
   - python: "3.6"
     env: TOXENV=py36
   - python: "3.7"
+    dist: xenial
     env: TOXENV=py37
-  - python: "3.7"
-    env: TOXENV=codestyle
-    before_install: true
 
 before_install:
   - wget "https://dist.ipfs.io/go-ipfs/v0.4.18/go-ipfs_v0.4.18_linux-amd64.tar.gz" -O /tmp/ipfs.tar.gz