project/plugins.sbt: update sbt-pgp to 0.8.1.

Change-Id: I167a73e65944873f7a67318faa8eb8d3877293e5
project/plugins.sbt: update sbt-idea to 1.5.1.

Change-Id: I8e34044239995e81e109c8c490666ca53301b82b
sbt: update to sbt 0.13.0.

The sbt-scalariform needed to be updated to 1.2.0 to build.

Change-Id: I554f04692194cd8a24295cb8f97fce0efe88b17d
Merge branch 'update-build-dependencies'

Change-Id: I1ef66738c20c282083c126ec43ed0e81458be4f5