diff --git a/.travis.yml b/.travis.yml
index dd8f22a2..a3667338 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,7 +3,7 @@ sudo: required
 services:
   - docker
 python:
-  - 3.5
+  - 3.4
 
 install:
   - pip install -e .
diff --git a/repo2docker/detectors.py b/repo2docker/detectors.py
index effbeddd..1ccbc0ab 100644
--- a/repo2docker/detectors.py
+++ b/repo2docker/detectors.py
@@ -352,7 +352,7 @@ class BuildPack(LoggingConfigurable):
 
     def build(self, image_spec):
         tarf = io.BytesIO()
-        tar = tarfile.open(fileobj=tarf, mode='x')
+        tar = tarfile.open(fileobj=tarf, mode='w')
         dockerfile_tarinfo = tarfile.TarInfo("Dockerfile")
         dockerfile = self.render().encode('utf-8')
         dockerfile_tarinfo.size = len(dockerfile)
diff --git a/setup.py b/setup.py
index c7110ed1..9bd72b42 100644
--- a/setup.py
+++ b/setup.py
@@ -4,7 +4,7 @@ from setuptools import setup, find_packages
 
 setup(
     name='jupyter-repo2docker',
-    version='0.2.10',
+    version='0.3',
     install_requires=[
         'docker',
         'traitlets',
@@ -12,13 +12,14 @@ setup(
         'escapism',
         'jinja2'
     ],
+    python_requires='>=3.4',
     author='Yuvi Panda',
     author_email='yuvipanda@gmail.com',
     license='BSD',
     packages=find_packages(),
     entry_points={
         'console_scripts': [
-            'jupyter-repo2docker = repo2docker.__main__:main'
+            'jupyter-repo2docker = repo2docker.__main__:main',
         ]
     },
 )