Disable signing of build artifacts

[cgtz]

By default signing artifacts with the bintray public key is enabled.
However, this seems to add about 30 minutes to the :bintrayPublish task,
which is too long for travis-ci builds.

Signed artifacts are not required unless consumers of the libraries are
interested in verifying the signature.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@3e619d0). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1260   +/-   ##
=========================================
  Coverage          ?   72.34%           
  Complexity        ?     6131           
=========================================
  Files             ?      440           
  Lines             ?    35225           
  Branches          ?     4478           
=========================================
  Hits              ?    25484           
  Misses            ?     8580           
  Partials          ?     1161

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