deps: update dependency com.google.guava:guava-bom to v29 (#95)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [com.google.guava:guava-bom](https://togithub.com/google/guava) | major | `28.2-android` -> `29.0-android` |

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pom.xml

@@ -159,7 +159,7 @@
     <grpc.version>1.28.1</grpc.version>
     <protobuf.version>3.11.4</protobuf.version>
     <junit.version>4.13</junit.version>
-    <guava.version>28.2-android</guava.version>
+    <guava.version>29.0-android</guava.version>
     <threeten.version>1.4.3</threeten.version>
     <opencensus.version>0.24.0</opencensus.version>
     <mockito.version>1.10.19</mockito.version>