Merge pull request #1216 from OCamlPro/fix-cudf-lock

Add dose patch for clean exit on solver errors
Merge pull request #1218 from OCamlPro/switch-import-solver-fix

Fixed an issue that could happen in some cases of `opam switch import`
Merge pull request #1225 from OCamlPro/travis-cohttp

Remove travis version constraint on cohttp
Add support for 'package flags' in opam files
Adding positions to the OPAM parse-tree

This should finally give us proper locations when anything gets wrong in
a file. I'm confident this will save people more time than it took me to
add :)
Be more lax on OPAM files errors

eg print the error but don't block. This will let people fix the
error and update again. Closes #1125