mark python<3.5 as unsupported (`pipenv sync` fails)

> Installing dependencies from Pipfile.lock (ea4cff)...
> Traceback (most recent call last):
>   File "/home/travis/virtualenv/python3.4.8/bin/pipenv", line 11, in <module>
>     sys.exit(cli())
>   File "/home/travis/virtualenv/python3.4.8/lib/python3.4/site-packages/pipenv/vendor/click/core.py", line 722, in __call__
>     return self.main(*args, **kwargs)
>   File "/home/travis/virtualenv/python3.4.8/lib/python3.4/site-packages/pipenv/vendor/click/core.py", line 697, in main
>     rv = self.invoke(ctx)
>   File "/home/travis/virtualenv/python3.4.8/lib/python3.4/site-packages/pipenv/vendor/click/core.py", line 1066, in invoke
>     return _process_result(sub_ctx.command.invoke(sub_ctx))
[...]
>  File "/home/travis/virtualenv/python3.4.8/lib/python3.4/site-packages/pipenv/core.py", line 1322, in do_init
>    pypi_mirror=pypi_mirror,
>  File "/home/travis/virtualenv/python3.4.8/lib/python3.4/site-packages/pipenv/core.py", line 807, in do_install_dependencies
>    pypi_mirror=pypi_mirror,
>  File "/home/travis/virtualenv/python3.4.8/lib/python3.4/site-packages/pipenv/core.py", line 1375, in pip_install
>    package_name.split('--hash')[0].split('--trusted-host')[0]
>  File "/home/travis/virtualenv/python3.4.8/lib/python3.4/site-packages/pipenv/vendor/requirementslib/models/requirements.py", line 704, in from_line
>    line, extras = _strip_extras(line)
>TypeError: 'module' object is not callable

https://travis-ci.org/fphammerle/freesurfer-surface/jobs/531434740

.travis.yml

@@ -1,7 +1,6 @@
 language: python
 
 python:
-- 3.4
 - 3.5
 - 3.6
 - 3.6-dev

setup.py

@@ -34,8 +34,6 @@ setuptools.setup(
         'Development Status :: 3 - Alpha',
         'Intended Audience :: Healthcare Industry',
         'Intended Audience :: Science/Research',
-        # TODO
-        # 'Programming Language :: Python :: 3.4',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
@@ -49,6 +47,7 @@ setuptools.setup(
             'freesurfer-annotation-labels = freesurfer_surface.__main__:annotation_labels',
         ],
     },
+    python_requires='>=3.5',
     install_requires=[
         'numpy<2',
     ],