SBT_PGP_USE_GPG is a system property, not an environment variable

[smarter]

The README says:

Or by setting SBT_PGP_USE_GPG environment variable to 0.

This is wrong on two counts:

• sbt-pgp/sbt-pgp/src/main/scala/com/jsuereth/sbtpgp/PgpSettings.scala

  Lines 22 to 25 in 90465fc

  	sys.props.get("SBT_PGP_USE_GPG") match {
  	case Some(_) => java.lang.Boolean.getBoolean("SBT_PGP_USE_GPG")
  	case None => true
  	}

• While setting the system property to 0 would indeed result in java.lang.Boolean.getBoolean("SBT_PGP_USE_GPG") returning false, the same would happen if it was set to 1. Boolean system properties should be set to either "false" or "true" to be properly parsed by java.lang.Boolean.getBoolean.