Modernize CI configuration

* Do not test with libc++. As ADOL-C is built using libstd++ we get strange
  linker and run-time errors.

* Mention that the problem with return value of the ADOL-C adouble
  comparison operators has been fixed in version 2.7.2.
  This will allow to eventually remove the code that patches dune-common
  for the CI system.

* Add gcc test with Dune 2.7.

* Remove the clang test with Dune 2.6.  It makes clang crash, but this
  case is not important enough to make me debug it.

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