travis-ci: build all branches instead of only master

So all new dev branches are built automatically without having to either
manually add the branch name to .travis.yml or being forced to create a
PR.

This creates redundant build on the same commit when PR is created but
otherwise no other harms.

No redundant build on PR merge since a brand new commit is created for
both merge commit or squash commit scenario.

See discussion
Ouranosinc/raven#292 (comment)

{{cookiecutter.project_repo_name}}/.travis.yml

@@ -5,9 +5,6 @@ os:
 python:
   - "3.6"
   - "3.7"
-branches:
-  only:
-    - master
 jobs:
   include:
     - os: osx