tox: Workaround double dep in with-sslib-master

Commit eb00d14 modified requirements-pinned.txt so that sslib specifiers
are now "[crypto,pynacl]". This happens to match the exact specifiers
used for the sslib git master dependency in tox.ini. This triggers pip
to say:
  ERROR: Double requirement given: securesystemslib[crypto,pynacl]==0.16.0
  (from -r /home/jku/src/tuf/requirements-pinned.txt (line 12)) (already
  in securesystemslib[crypto,pynacl] from
  git+http://github.com/secure-systems-lab/securesystemslib.git@master#egg=securesystemslib[crypto,pynacl],
  name='securesystemslib')

Avoid this by not setting any specifiers for the sslib git master
dependency in tox.ini: This makes pip happy and we get the git master
version installed. pynacl and crypto are still installed because they
are in requirements-pinned.txt.

Fixes #1184.

Signed-off-by: Jussi Kukkonen <jkukkonen@vmware.com>

Author: Jussi Kukkonen

tox.ini

@@ -30,7 +30,7 @@ install_command = pip install --pre {opts} {packages}
 # Must to be invoked explicitly with, e.g. `tox -e with-sslib-master`
 [testenv:with-sslib-master]
 deps =
-    --editable git+http://github.com/secure-systems-lab/securesystemslib.git@master#egg=securesystemslib[crypto,pynacl]
+    --editable git+http://github.com/secure-systems-lab/securesystemslib.git@master#egg=securesystemslib
     -r{toxinidir}/requirements-test.txt
     --editable {toxinidir}