MNT: Update requirements.txt post-1.4

[effigies]

Summary

While looking at the Travis failure, I realized we were still installing things intended for Python 2 and 3.4.

This adds a tools/update_requirements.py file to make keeping the requirements file in sync with the setup configuration easy.

List of changes proposed in this PR (pull-request)

Acknowledgment

• (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[codecov]

Codecov Report

Merging #3153 into maint/1.4.x will decrease coverage by 0.31%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           maint/1.4.x    #3153      +/-   ##
===============================================
- Coverage        67.59%   67.28%   -0.32%     
===============================================
  Files              299      299              
  Lines            39499    39499              
  Branches          5220     5220              
===============================================
- Hits             26700    26576     -124     
- Misses           12086    12182      +96     
- Partials           713      741      +28

Flag	Coverage Δ
#smoketests	52.06% <ø> (-0.97%)	⬇️
#unittests	64.79% <ø> (-0.05%)	⬇️

Impacted Files	Coverage Δ
nipype/interfaces/dcm2nii.py	49.76% <0%> (-8.93%)	⬇️
nipype/utils/config.py	60.77% <0%> (-4.42%)	⬇️
nipype/pipeline/engine/utils.py	76.96% <0%> (-3.54%)	⬇️
nipype/interfaces/fsl/model.py	77.87% <0%> (-2.71%)	⬇️
nipype/interfaces/fsl/preprocess.py	79.41% <0%> (-2.25%)	⬇️
nipype/interfaces/io.py	58.35% <0%> (-1.32%)	⬇️
nipype/interfaces/fsl/base.py	85.39% <0%> (-1.13%)	⬇️
nipype/pipeline/engine/workflows.py	76.44% <0%> (-0.34%)	⬇️
nipype/interfaces/base/core.py	86.77% <0%> (-0.2%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7dd3b37...496299e. Read the comment docs.

[effigies]

This also fixes the Travis failures (somewhat unsatisfyingly) by using --upgrade. This isn't great because it doesn't help catch cases where upstream pre-releases are breaking things for older versions of other libraries, but that's also not our responsibility, and we do want to check the latest releases of all libraries.