Update Travis config for broken snapd on ppc (#7381)

Summary:
snapd update has been failing on ppc for ~a week. Disabling it
for now in pull requests.

Also, https://github.com/facebook/rocksdb/issues/6653 seems to be fixed, so re-enabling standard unit tests for
PPC on pull requests.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7381

Test Plan: CI

Reviewed By: jay-zhuang

Differential Revision: D23684962

Pulled By: pdillinger

fbshipit-source-id: 96ec9487b714c4741bb1653dae90b24118830cb5

.travis.yml

@@ -100,11 +100,11 @@ matrix:
     arch: amd64
     env: JOB_NAME=cmake-gcc9
   # 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)
+  # Temporarily disable ppc64le cmake test while snapd is broken
   - if: type = pull_request AND commit_message !~ /FULL_CI/
     os: linux
     arch: ppc64le
-    env: TEST_GROUP=platform_dependent
+    env: JOB_NAME=cmake
   # NB: the cmake build is a partial java test
   - if: type = pull_request AND commit_message !~ /FULL_CI/
     os: osx