Fix doctests and update CI validation

[k0ral]

The aim of this pull-request is to:

• fix https://travis-ci.org/snoyberg/xml/jobs/644883007
• clean-up and update CI validation:
  • Stackage LTS 7 to 10 are not supported as is, because of multiple dependencies' lower bounds
  • Stackage LTS 9 fails to validate doctests, for some reason
  • Stackage LTS 13 and 14 should be validated
  • latest versions of GHC 8.0.*, 8.2.*, 8.4.* and 8.6.* should be validated