Restore to Stackage LTS after GHC 8.6.1

[orome]

Resolve two related issues preventing incorporation into LTS after GHC 8.6.1:

• fix package build constraint on base limiting to GHC < 8.6.1, and
• confirm update to MissingH that causes this dependency to be excluded from LTS.

Finally,

• confirm restoring of package to Stackage build-constraints.yaml,
• confirm package builds without MissingH as an extra-dep, and
• confirm that package has been restored to LTS nightly.

Fix package constraint on base

Fixed by simple relaxation of constraints in .cabal.

Confirm update to MissingH that causes this dependency to be excluded from LTS

Not strictly a change to the package, but rather a change to the required MissingH package, involving relaxing build constraints and fixing an issue with Control.Monad.Fail.MonadFail, fixed by MissingH maintainers.

Note that MissingH still fails its tests, but this does not exclude it from LTS, so that it now builds for LTS.

The longer term solution to this is to remove the dependency on MissingH by using a different package (#29).

Confirm restoring of package to Stackage build-constraints.yaml

Done: commercialhaskell/stackage@ad838fc.

Confirm package builds without MissingH as an extra-dep

Confirmed Travis build of 2e214b7

Confirm that package has been restored to LTS nightly

Confirmed for nightly-2018-10-16!

[orome]

Restored in nightly-2018-10-16.