Use native strs for build failure error messages

codeship-steps.yml

@@ -1,3 +1,7 @@
+- name: clean-build
+  service: avirshup/docker-make
+  command: sh -c "rm -f /opt/distfiles/*"
+
 - name: build-pkg
   service: avirshup/docker-make
   command: sh -c 'cp -v /opt/DockerMake/dist/DockerMake-*.tar.gz /opt/distfiles/'
@@ -9,7 +13,7 @@
     - testenv-python3.5
     - testenv-python3.6
   steps:
-    - command: sh -c "pip install /opt/distfiles/DockerMake-*.tar.gz && py.test"
+    - command: sh -c "pip install /opt/distfiles/DockerMake-*.tar.gz && py.test -v"
 
 - name: deploy-dockerhub
   type: push

dockermake/errors.py

@@ -13,9 +13,10 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-from io import StringIO
+import io
 import pprint
 from termcolor import cprint
+from future.utils import PY2
 
 class UserException(Exception):
     """
@@ -84,12 +85,14 @@ class BuildError(Exception):
     def __init__(self, dockerfile, item, build_args):
         with open('dockerfile.fail', 'w') as dff:
             print(dockerfile, file=dff)
-        with StringIO() as stream:
+
+        buffer_class = io.BytesIO if PY2 else io.StringIO
+        with buffer_class() as stream:
             cprint('Docker build failure', 'red', attrs=['bold'], file=stream)
-            print(u'\n   -------- Docker daemon output --------', file=stream)
+            print('\n   -------- Docker daemon output --------', file=stream)
             pprint.pprint(item, stream, indent=4)
-            print(u'   -------- Arguments to client.build --------', file=stream)
+            print('   -------- Arguments to client.build --------', file=stream)
             pprint.pprint(build_args, stream, indent=4)
-            print(u'This dockerfile was written to dockerfile.fail', file=stream)
+            print('This dockerfile was written to dockerfile.fail', file=stream)
             stream.seek(0)
             super(BuildError, self).__init__(stream.read())