travis-build: also run javadoc generation for PRs

[imagejan]

It seems that the maven-javadoc-plugin is not executed when Building the artifact locally only, i.e. for Travis builds that are triggered by open PRs.

See for example: https://travis-ci.org/ijpb/MorphoLibJ/builds/364542659

[imagejan]

There seems to be consensus that javadoc generation should not happen when building locally from the CLI (because it can take long), but it should always happen when building artifacts with TravisCI.

See also this discussion on Gitter.

[imagejan]

Presumably fixed by @ctrueden in 69d29a5.

Thanks!