switch default branch from develop to master

[hohwille]

As a devonfw maintainer, I want devon4j to use master as the default branch rather than develop so that we have this consistent accross all repositories in devonfw.
The default branch in devon4j was changed to develop a long time ago because we also had release branches like develop-3.1.x. As we have changed to a linear and fully PR based development there is no need for this anymore.
However, not using master is causing some issues and non-standards compared to other devonfw repos:

• #294 : Sonar integration with devon4j #300 (comment)

• devon4j/pom.xml

  Line 30 in 0a16e2a

  <github.default.branch>develop</github.default.branch>

• devon4j/.travis.yml

  Line 29 in 0a16e2a

  - mvn install && ([[ $TRAVIS_BRANCH != "develop" ]] || [[ $TRAVIS_PULL_REQUEST != "false" ]] || mvn --settings .mvn/settings.xml -DskipTests=true -Darchetype.test.skip=true deploy)

• devon4j/.travis.yml

  Line 31 in 0a16e2a

  if: (branch = develop) AND (type = push)

[hohwille]

See also:
https://github.com/github/renaming#creating-a-seamless-move-for-existing-repositories-

[hohwille]

Done.

[hohwille]

I also deleted the old branches such as develop, develop-3.1.x, etc. so we have a clean repo now which follows our conventions we have for all other repos.