Run system test using sauce labs on travis

[gdetrez]

This adds an extra stage to the travis build that runs the system tests using saucelabs browsers.

• it is configured to use two browser: Firefox and Chrome, both on windows (the available browsers on Linux are lagging behind quite a bit :disappointed:).
• It will only run on push to master. There are a few reason for this: the credentials are not available on external pull requests ; I want to make sure it's not too flaky ; and more importantly, it takes 20 minutes to run :fearful:

[codecov-io]

Codecov Report

Merging #370 into master will decrease coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #370      +/-   ##
==========================================
- Coverage   40.26%   40.24%   -0.02%     
==========================================
  Files         195      195              
  Lines       16716    16716              
  Branches     2921     2921              
==========================================
- Hits         6730     6728       -2     
+ Misses       9571     9552      -19     
- Partials      415      436      +21

Impacted Files	Coverage Δ
scanomatic/data_processing/phases/segmentation.py	73.53% <0%> (-1.12%)	:arrow_down:
scanomatic/ui_server/tools_api.py	8.77% <0%> (ø)	:arrow_up:
scanomatic/ui_server/general.py	22.41% <0%> (ø)	:arrow_up:
scanomatic/data_processing/phenotyper.py	27.51% <0%> (ø)	:arrow_up:
scanomatic/io/source.py	18.08% <0%> (ø)	:arrow_up:
scanomatic/ui_server/data_api.py	9.65% <0%> (ø)	:arrow_up:
scanomatic/qc/phenotype_results.py	0% <0%> (ø)	:arrow_up:
scanomatic/data_processing/phases/features.py	17.23% <0%> (ø)	:arrow_up:
scanomatic/image_analysis/grid.py	76.69% <0%> (+0.75%)	:arrow_up:

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