Upgrade Guava to 30.0-jre and appengine-plugins-core to 0.9.5

[chanseokoh]

$ git grep '28\.1' shows nothing relevant while $ git grep '30\.0' shows these changes.

[chanseokoh]

Let's see if tests pass.

[chanseokoh]

Maybe they added @DoNotMock to recent Guava versions. I don't see an easy way to get around it for our test usage, so suppressing it.

1461695 [ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.0:compile (default) on project com.google.cloud.tools.eclipse.googleapis.test: Compilation failure: Compilation failure: 
1461696 [ERROR] /home/travis/build/GoogleCloudPlatform/google-cloud-eclipse/plugins/com.google.cloud.tools.eclipse.googleapis.test/src/com/google/cloud/tools/eclipse/googleapis/internal/GoogleApiFactoryTest.java:[59,56] [DoNotMock] Do not mock 'com.google.common.cache.LoadingCache<com.google.cloud.tools.eclipse.googleapis.internal.GoogleApi,com.google.api.client.http.HttpTransport>' (which is-a 'com.google.common.cache.Cache'); com.google.common.cache.Cache is annotated as @DoNotMock: Use CacheBuilder.newBuilder().build().
1461699 [ERROR]     (see https://errorprone.info/bugpattern/DoNotMock)
1461699 [ERROR] /home/travis/build/GoogleCloudPlatform/google-cloud-eclipse/plugins/com.google.cloud.tools.eclipse.googleapis.test/src/com/google/cloud/tools/eclipse/googleapis/internal/GoogleApiFactoryWithProxyServerTest.java:[52,56] [DoNotMock] Do not mock 'com.google.common.cache.LoadingCache<com.google.cloud.tools.eclipse.googleapis.internal.GoogleApi,com.google.api.client.http.HttpTransport>' (which is-a 'com.google.common.cache.Cache'); com.google.common.cache.Cache is annotated as @DoNotMock: Use CacheBuilder.newBuilder().build().
1461702 [ERROR]     (see https://errorprone.info/bugpattern/DoNotMock)

[briandealwis]

LGTM