This updates the plugin to work with both Babel 7 and Babel 8:

Declared @babel/core ^7.0.0 || ^8.0.0 as a peer dependency.
Updated dev dependencies to Babel 8 RC for testing.
Fixed t.Identifier() to use the canonical t.identifier() builder.
Added a plugin name for improved Babel 8 error messages.
Replaced _gettextTransformed node mutation with a WeakSet.

Ran unit tests against both Babel 7.x and 8.0.4