Error Prone 2.0.7 release

[Liam Miller-Cushon]

We are pleased to announce the release of Error Prone 2.0.7.

New checks:

* [BadComparable] Possible sign flip from narrowing conversion

* [MissingOverride] Missing @Override annotation

* [InsecureCipherMode] Insecure usage of Cipher.getInstance

* [Immutable] Type declaration annotated with @Immutable is not immutable

* [MislabeledAndroidString] Use of mislabeled resource in `android.R.string`

Closed issues:

* CheckReturnValue and ReturnValueIgnored should disable checking on mocked objects (#226)