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:

• https://github.com/sbt/sbt-pgp/blob/90465fcde014eb0688f0086fdec56db60ebfbfd6/sbt-pgp/src/main/scala/com/jsuereth/sbtpgp/PgpSettings.scala#L22-L25
• 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.