Added devel job that runs with unreleased molecule

Enables testing with latest master code of molecule so we avoid
merging changes that may not be compatible with it.

Author: ssbarnea

.travis.yml

@@ -25,6 +25,9 @@ jobs:
     - name: py37
       python: "3.7"
       env: TOXENV=py37
+    - name: devel
+      python: "3.7"
+      env: TOXENV=devel
     - stage: deploy
       name: deploy
       if: tag IS present AND type != cron

tox.ini

@@ -1,6 +1,6 @@
 # For more information about tox, see https://tox.readthedocs.io/en/latest/
 [tox]
-envlist = lint,py27,py35,py36,py37
+envlist = lint,py27,py35,py36,py37,devel
 
 [testenv]
 deps =
@@ -30,6 +30,11 @@ whitelist_externals =
     pytest
     pre-commit
 
+[testenv:devel]
+commands =
+    pip install git+https://github.com/ansible/molecule#egg=molecule
+    {[testenv]commands}
+
 [testenv:lint]
 skip_install = true
 deps =