Update .travis.yml

[ilarimikkonen]

removing build group definition, since we get problems with docker build: Failed to fetch http://deb.debian.org/debian/dists/jessie-updates/InRelease Unable to find expected entry 'main/binary-amd64/Packages' in Release file (Wrong sources.list entry or malformed file) Idea is to see if default Travis build env performs better

Closes #

Changes

Describe your changes here as a bulleted list:

• change one
• change two

Developer checklist

This checklist is to be completed by the PR developer:

• [ ] Alternative solutions were compared/discussed before writing code
  • [ ] trade-offs with this solution are considered acceptable
• [ ] Code in this PR adheres to the project styleguide
• [ ] This pull request does not decrease project test coverage
• [ ] If the code changes existing database collection(s), migration has been written
• [ ] If UI texts are added or changed, all texts are internationalized

Reviewer checklist

Reviewed by: @username1

This list is to be completed by the pull request reviewer:

• [ ] Code works as described/expected
• [ ] Code seems to be error free
  • [ ] no browser console errors visible
  • [ ] no server console errors visible
  • [ ] passes CI build
• [ ] Code is written in a way that promotes maintainability
  • [ ] easy to understand
  • [ ] well organized
  • [ ] follows project coding standards and conventions
  • [ ] well documented