Only support dune 2.7 and higher

@osander: Can you have a look here? Thanks!

added 1 commit

• 7fadb69f - Only support dune-elasticity >=2.7

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added 6 commits

• 297d851d - Remove ci-tests for dune-2.6, as we only want to supprt dune 2.7 and higher from now on
• 780368c8 - Only support dune-functions >=2.7
• 1cab5df0 - Only support dune-geometry >=2.7
• 778e12e7 - Only support dune-common >=2.7
• e913ae37 - Only support dune-grid >=2.7
• 3aa5527c - Only support dune-elasticity >=2.7

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merged

mentioned in commit a65869ba

Thanks!

I squashed the commits when merging -- watch out when you pull the changes.