Try 3.7-dev in CI

Use https over ssh when cloning go-libp2p-daemon

To avoid """
Warning: Permanently added the RSA host key for IP address '140.82.113.4' to the list of known hosts.
Permission denied (publickey).
fatal: Could not read from remote repository.
"""

Exit if clone fails

Fix daemon url

.travis.yml

@@ -2,13 +2,13 @@ language: python
 
 matrix:
   include:
-    - python: 3.7
+    - python: 3.7-dev
       dist: xenial
       env: TOXENV=py37-test
-    - python: 3.7
+    - python: 3.7-dev
       dist: xenial
       env: TOXENV=lint
-    - python: 3.7
+    - python: 3.7-dev
       dist: xenial
       env: TOXENV=py37-interop
       sudo: true