Update to Babel 7.0.0 (stable)

[MattiasBuelens]

Babel 7.0.0 has been released, so the new version of this plugin can also go out of beta. 😄

This PR prepares the plugin for a 8.0.0 release:

• Drop support for Babel 7 beta and rc releases
• Use Babel 7.0.0 for testing
• Update examples for Babel 7

Before the new release, we should also update the README to suggest using @6 for Babel 6:

Installation

With babel 7.x:

npm install karma-babel-preprocessor @babel/core @babel/preset-env --save-dev

With babel 6.x:

npm install karma-babel-preprocessor@6 babel-core babel-preset-env --save-dev