Add python-dateutil to test requirements

tests/test_api.py uses python-dateutil, therefore ensure it is installed
for Travis and AppVeyor.

Signed-off-by: Joshua Lock <jlock@vmware.com>

Author: joshuagl

appveyor.yml

@@ -27,7 +27,7 @@ install:
   - set PATH=%PYTHON%;%PYTHON%\\Scripts;%PATH%
   - python -m pip install -U pip setuptools
   - pip install -e .
-  - pip install securesystemslib[crypto,pynacl]
+  - pip install securesystemslib[crypto,pynacl] python-dateutil
   - if %PYTHON_VERSION%==2.7 pip install mock
 
 build: false

requirements-test.txt

@@ -6,6 +6,9 @@
 # test runtime dependencies  (see 'tests_require' field in setup.py)
 mock; python_version < "3.3"
 
+# tuf.api tests use python-dateutil
+python-dateutil
+
 # additional test tools for linting and coverage measurement
 coverage
 pylint