[Test fix] removing minHeapSize and increasing maxHeapSize to 4g

[cameronlee314]

Symptom: CI build fails transiently with an error that looks like:

Execution failed for task ':samza-core_2.11:test'.
> Process 'Gradle Test Executor 6' finished with non-zero exit value 1
  This problem might be caused by incorrect test process configuration.
  Please refer to the test execution section in the User Manual at https://docs.gradle.org/5.2.1/userguide/java_testing.html#sec:test_execution

Cause: Recently, we upgraded to Gradle 5. Gradle 5 lowered some memory allocation defaults (https://docs.gradle.org/5.0/userguide/upgrading_version_4.html#rel5.0:default_memory_settings), and it looks like the lower memory isn't sufficient for some tests. We tried using 3g of memory in #1307, but it still looks like that is not enough.
Changes: Increase the memory for running tests for samza-core, which seems to be the impacted module. The minHeapSize configuration was also removed in the hope that it allows memory usage to be automatically managed better.
Tests: CI build passed (https://travis-ci.org/apache/samza/builds/660834705?utm_source=github_status&utm_medium=notification)