MNT: Bump Python dependency to 3.5

The latest PyYAML release (5.3) has dropped support for Python 3.4,
putting our 3.4 build on Travis into a failing state [0].  When we
transitioned to Python 3 only, we didn't have a strong reason for
choosing 3.4 as our floor (discussed in gh-369), so let's just bump it
to 3.5.

[0]: https://travis-ci.org/ReproNim/reproman/jobs/653138599

.travis.yml

@@ -38,7 +38,7 @@ matrix:
     env:
     - REPROMAN_LOGLEVEL=2
     - REPROMAN_LOGTARGET=/dev/null
-  - python: 3.4
+  - python: 3.6
     # Note: This this no network run appears to hang or otherwise
     # behave oddly on 3.5.
     env:

CONTRIBUTING.md

@@ -113,7 +113,7 @@ we outline the workflow used by the developers:
 Development environment
 -----------------------
 
-We support Python 3 (>= 3.4).
+We support Python 3 (>= 3.5).
 
 See [README.md:Dependencies](README.md#Dependencies) for basic information
 about installation of reproman itself.

README.md

@@ -27,7 +27,7 @@ internals and/or contributing to the project.
 
 # Installation
 
-ReproMan requires Python 3 (>= 3.4).
+ReproMan requires Python 3 (>= 3.5).
 
 ## Debian-based systems
 

setup.py

@@ -144,7 +144,7 @@ def findsome(subdir, extensions):
     description="Neuroimaging Computational Environments Manager",
     long_description=long_description,
     packages=reproman_pkgs,
-    python_requires='>=3.4',
+    python_requires='>=3.5',
     install_requires=requires['core'],
     extras_require=requires,
     entry_points={