Update travis to xenial and misc updates

- Add a build matrix to run each tox env in a corresponding
  travis env as per travis/tox best practices.
  https://docs.travis-ci.com/user/languages/python/#using-tox-as-the-build-script
- Add Python 3.5 tests
- Remove only build on certain branch restrictions
- Use "install" instead of "before_script"  to install dependencies.
  Explicitly listing "install" prevents Travis from automatically
  running `pip install -r requirements.txt`, which is not necessary
  because most of those requirements are installed again in each
  tox environment.
- Move pylint and bandit calls to tox (pylint requires
  dependencies) to be installed.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

.travis.yml

@@ -1,29 +1,25 @@
+dist: xenial
 language: python
-dist: trusty
-sudo: false
 cache: pip
-python: 3.6
 
-env:
-  - TOXENV=py27
-  - TOXENV=py34
-  - TOXENV=py36
+matrix:
+  include:
+    - python: "2.7"
+      env: TOXENV=py27
+    - python: "3.4"
+      env: TOXENV=py34
+    - python: "3.5"
+      env: TOXENV=py35
+    - python: "3.6"
+      env: TOXENV=py36
 
-before_script:
-  - pip install pylint bandit tox coveralls
+install:
+  - pip install tox coveralls
 
 script:
-  - pylint tuf
-  - bandit -r tuf
   - tox
 
 after_success:
   - cd tests
   - coveralls
   - cd -
-
-branches:
-  only:
-    - develop
-    - pylint
-    - bandit

ci-requirements.txt

@@ -2,6 +2,6 @@ securesystemslib[crypto,pynacl]
 six
 iso8601
 coverage
-coveralls
 pylint
+bandit
 requests

tox.ini

@@ -10,6 +10,8 @@ envlist = py27, py34, py35, py36
 changedir = tests
 
 commands =
+    pylint {toxinidir}/tuf
+    bandit -r {toxinidir}/tuf
     coverage run --source tuf aggregate_tests.py
     coverage report -m --fail-under 97