travis: Fix failing master branch

[kyleam]

Fix the failing Travis build with master by (1) bumping our required Python version to 3.5 and (2) using a released version of DataLad.

Closes #503

[codecov[bot]]

Codecov Report

Merging #504 into master will decrease coverage by 0.13%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #504      +/-   ##
==========================================
- Coverage   89.66%   89.52%   -0.14%     
==========================================
  Files         148      148              
  Lines       12209    12209              
==========================================
- Hits        10947    10930      -17     
- Misses       1262     1279      +17     

Impacted Files	Coverage Δ
reproman/distributions/tests/test_docker.py	100.00% <0.00%> (ø)	:arrow_up:
reproman/resource/tests/test_ssh.py	100.00% <0.00%> (ø)	:arrow_up:
reproman/distributions/tests/test_venv.py	87.50% <0.00%> (-10.58%)	:arrow_down:
reproman/interface/tests/test_execute.py	100.00% <0.00%> (ø)	:arrow_up:
reproman/interface/execute.py	94.90% <0.00%> (ø)	:arrow_up:
reproman/support/jobs/tests/test_orchestrators.py	93.44% <0.00%> (ø)	:arrow_up:
reproman/distributions/redhat.py	94.51% <0.00%> (ø)	:arrow_up:
reproman/resource/ssh.py	88.33% <0.00%> (-0.84%)	:arrow_down:
reproman/support/jobs/orchestrators.py	91.84% <0.00%> (ø)	:arrow_up:
reproman/distributions/conda.py	94.16% <0.00%> (ø)	:arrow_up:
... and 20 more

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[kyleam]

The main point of discussion here would be the minimum Python version dump, I think. That came up on the call yesterday, and there didn't seem to be any objections, so I'll merge this in.