Update securesystemslib dependency

We need the recently released securesystemslib 0.15.0 or newer for
abstract storage support.

Signed-off-by: Joshua Lock <[email protected]>

Author: joshuagl

requirements-pinned.txt

@@ -11,7 +11,7 @@ pycparser==2.20           # via cffi
 pynacl==1.3.0             # via securesystemslib
 python-dateutil==2.8.1    # via securesystemslib
 requests==2.23.0
-securesystemslib[colors,crypto,pynacl]==0.14.2
+securesystemslib[colors,crypto,pynacl]==0.15.0
 six==1.14.0
 subprocess32==3.5.4       ; python_version < '3' # via securesystemslib
 urllib3==1.25.9           # via requests

setup.py

@@ -116,7 +116,7 @@
     'iso8601>=0.1.12',
     'requests>=2.19.1',
     'six>=1.11.0',
-    'securesystemslib>=0.12.0'
+    'securesystemslib>=0.15.0'
   ],
   tests_require = [
     'mock; python_version < "3.3"'