Temporarily disable ppc64le unit tests in PRs (#6682)

Summary:
Until Travis gets its act together (https://github.com/facebook/rocksdb/issues/6653)
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6682

Test Plan: CI

Reviewed By: riversand963

Differential Revision: D20948865

Pulled By: pdillinger

fbshipit-source-id: 215de523c91a83d2a159f466b853e700c925ba4f

.travis.yml

@@ -71,6 +71,11 @@ matrix:
   - os: linux
     compiler: clang
   # Exclude most osx, arm64 and ppc64le tests for pull requests, but build in branches
+  # Temporarily disable ppc64le unit tests in PRs until Travis gets its act together (#6653)
+  - if: type = pull_request
+    os: linux
+    arch: ppc64le
+    env: TEST_GROUP=platform_dependent
   # NB: the cmake build is a partial java test
   - if: type = pull_request
     os: osx