Fix some testing

.travis.yml

@@ -1,4 +1,4 @@
-sudo: false
+os: linux
 language: python
 cache: pip
 dist: xenial

stix2/core.py

@@ -325,11 +325,8 @@ def _register_observable_extension(
     EXT_MAP = STIX2_OBJ_MAPS[v]['observable-extensions']
 
     try:
-        try:
-            if ext_type in EXT_MAP[observable_type].keys():
-                raise DuplicateRegistrationError("Observable Extension", ext_type)
-        except AttributeError:
-            pass
+        if ext_type in EXT_MAP[observable_type].keys():
+            raise DuplicateRegistrationError("Observable Extension", ext_type)
         EXT_MAP[observable_type][ext_type] = new_extension
     except KeyError:
         if observable_type not in OBJ_MAP_OBSERVABLE:

stix2/test/v20/test_custom.py

@@ -1080,8 +1080,7 @@ def test_register_duplicate_observable_with_version():
     assert "cannot be registered again" in str(excinfo.value)
 
 
-@pytest.mark.xfail(reason="The default version is no longer 2.0", condition=stix2.DEFAULT_VERSION != "2.0")
-def test_register_marking_with_no_version():
+def test_register_marking_with_version():
     @stix2.v20.CustomMarking(
         'x-new-obj-2', [
             ('property1', stix2.properties.StringProperty(required=True)),

tox.ini

@@ -8,11 +8,11 @@ deps =
   pytest
   pytest-cov
   coverage
-  taxii2-client
+  taxii2-client<1.0.0
   fuzzywuzzy
   haversine
   python-Levenshtein
-  medallion
+  medallion<2.0.0
 commands =
   python -m pytest --cov=stix2 stix2/test/ --cov-report term-missing -W ignore::stix2.exceptions.STIXDeprecationWarning