Bumb sbt-pgp version to 0.8.1, corrects "dispatch" version

The sbt-pgp plugin v0.8 contained an incorrect selection
of the dispatch library version (the _2.9.1 version is
selected). That also has the effect of making it
incompatible with dbuild. Fixed in sbt-pgp 0.8.1.
Merge pull request #5 from cunei/master

Bumb sbt-pgp version to 0.8.1, corrects "dispatch" version