Test failure on master branch: GeoClaw multi-layer basic plane-waves

[ketch]

See https://travis-ci.org/clawpack/clawpack/jobs/376683975. This isn't causing a failure in the GeoClaw repo -- do we just need to update the version that Clawpack points to?

ERROR: Test multi-layer basic plane-waves.
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Traceback (most recent call last):
  File "/home/travis/build/clawpack/clawpack/geoclaw/tests/multilayer/regression_tests.py", line 42, in setUp
    super(MultilayerTest, self).setUp()
  File "/home/travis/build/clawpack/clawpack/clawpack/clawutil/test.py", line 164, in setUp
    self.build_executable()
  File "/home/travis/build/clawpack/clawpack/clawpack/geoclaw/test.py", line 53, in build_executable
    executable_name=executable_name)
  File "/home/travis/build/clawpack/clawpack/clawpack/clawutil/test.py", line 187, in build_executable
    raise e
CalledProcessError: Command 'cd /home/travis/build/clawpack/clawpack/geoclaw/tests/multilayer ; make .exe' returned non-zero exit status 2

[mandli]

That would be my guess. There was a commit that added AMR to the multilayer code and it changed some things that required changes also to Riemann. @aks2203 may be able to also take a look as that was primarily his PR.