Error Prone 2.0.8 release

[Arnaud Venet]

We are pleased to announce the release of Error Prone 2.0.8.

New checks:

- [TypeParameterQualifier]: forbids type parameters from being used as name qualifiers.

- [NullableConstructor]: constructors should be annotated with @Nullable since they cannot return null.

- [NullableVoid]: void-returning methods should not be annotated with @Nullable, since they cannot return null.

- [RectIntersectReturnValueIgnored]: flags usages of android.graphics.Rect.intersect() where the return value is ignored.

- [EqualsIncompatibleType]: flags usages of the default implementation of equals() with objects of incompatible types.

- [Immutable]: enforces usage of @Immutable.

- [Var]: introduces a @Var annotation and checks that it is applied to non-final variables.

Other changes:
- Fixes removeImport suggestion.

- Adds an overload for Matchers.methodReturns that accepts a Matcher.

- Adds support for com.ibm.icu.text.SimpleDateFormat to MisusedWeekYear checker.

- Migrates from deprecated isAssignableFrom to equivalent isSupertypeOf.

- Fixes bug in GuiceOverridesGuiceInjectableMethod.

- Makes DescendantOf less null-hostile.

- Relaxes UnsynchronizedOverridesSynchronized for InputStream.

- Relaxes CheckReturnValue in tests.

Fixes issues: #371, #378, #385.