Deprecate fail and failNel

As discussed in #720, `failure` and `failureNel` are preferred.
Merge pull request #722 from ceedubs/deprecate-fail

Deprecate fail and failNel
update sbt and sbt-unidoc plugin

sbt:
0.13.2 ->
0.13.5

sbt-unidoc:
0.3.0 ->
0.3.1
Merge pull request #723 from cosmo0920/update-sbt_0_13_5

update to sbt 0.13.5 and sbt-unidoc plugin
remove unused imports