Build and test against Error Prone 2.15.0

[msridhar]

EP 2.15.0 has a check enabled by default that we want to disable (see google/error-prone#3366). We cannot just disable this check with the existing config, since it does not exist in version 2.10.0, the latest version that works with JDK 8. Rather than adding conditional config around this one check, this PR disables compiling our code with Error Prone checks on JDK 8. We still check all our code with Error Prone on JDK 11+, and we are still running our own tests on JDK 8 (for now; see #634).

Beyond the config changes, no code changes were required.

[coveralls]

Pull Request Test Coverage Report for Build #966

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.04%) to 92.552%

---

Files with Coverage Reduction	New Missed Lines	%
../nullaway/src/main/java/com/uber/nullaway/dataflow/AccessPath.java	2	95.77%

Totals
Change from base Build #964:	-0.04%
Covered Lines:	4933
Relevant Lines:	5330

---

💛 - Coveralls

[ketkarameya]

LGTM.