Fix trailing slash on directories windows bug

[rok-cesnovar]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Fixes #281

Originally planned to do this as part of #289 but that might take a bit longer to polish so we can fix this here

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Rok Češnovar, Univ. of Ljubljana

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[codecov-commenter]

Codecov Report

Merging #294 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #294   +/-   ##
=======================================
  Coverage   90.11%   90.12%           
=======================================
  Files          12       12           
  Lines        2419     2421    +2     
=======================================
+ Hits         2180     2182    +2     
  Misses        239      239           

Impacted Files	Coverage Δ
R/utils.R	70.78% <ø> (ø)
R/args.R	99.76% <100.00%> (+<0.01%)	⬆️
R/install.R	55.38% <100.00%> (ø)
R/model.R	88.21% <100.00%> (+0.03%)	⬆️

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[jgabry]

Looks good!