TOX: Install securesystemslib in non-editable mode

Installing securesystemslib in editable mode leads to a problem
in sys.path where we have two "tests" packages.
By not installing securesystemslib in an editable mode we are not
adding the securesystemslib tests to sys.path.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

Author: MVrachev

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}