Merge pull request #1228 from MVrachev/fix-ci

TOX: Install securesystemslib in non-editable mode

Author: lukpueh

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
+    git+http://github.com/secure-systems-lab/securesystemslib.git@master#egg=securesystemslib
     -r{toxinidir}/requirements-test.txt
     --editable {toxinidir}