deps: downgrade opencensus version back to 0.28.0 (#265)

To address linkage checker error brought in by grpc-census identified in #248 (review),

Author: stephaniewang526

pom.xml

@@ -70,7 +70,7 @@
     <javax.annotations.version>1.3.2</javax.annotations.version>
     <animal-sniffer.version>1.19</animal-sniffer.version>
     <iam.version>1.0.7</iam.version>
-    <opencensus.version>0.28.1</opencensus.version>
+    <opencensus.version>0.28.0</opencensus.version>
     <findbugs.version>3.0.2</findbugs.version>
     <errorprone.version>2.5.1</errorprone.version>
     <jackson.version>2.12.1</jackson.version>