Fix very minor packaging issues

- State in README.rst and setup.py metainfo that we require Python>=3.5

- Let Travis-CI test against newer versions of Python

- Let instaloader --help show where to report issues

.travis.yml

@@ -2,7 +2,9 @@ language: python
 python:
  - "3.5"
  - "3.5-dev"
+ - "3.6"
  - "3.6-dev"
+ - "3.7-dev"
  - "nightly"
 install:
  - pip install pylint requests