Fix nexus publishing on travis

[larsgrefer]

fixes #573

see also: marcphilipp/nexus-publish-plugin#1

[codecov]

Codecov Report

Merging #574 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #574   +/-   ##
=========================================
  Coverage     91.12%   91.12%           
  Complexity      327      327           
=========================================
  Files            56       56           
  Lines           935      935           
  Branches        115      115           
=========================================
  Hits            852      852           
  Misses           60       60           
  Partials         23       23

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[persapiens]

In general, I prefer using release versions because of reproducibility. However, we don't lose it because you use one specific build version.