Don't preinstall jemalloc in Travis

Summary: Since we enabled jemalloc for open source builds, Travis looks like it's dying. Don't install jemalloc when running in travis

Test Plan: none

Reviewers: yhchiang

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D37659

.travis.yml

@@ -5,7 +5,7 @@ before_install:
 # which needs the following ugly dependency incantations to build RocksDB:
  - sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
  - sudo apt-get update -qq
- - sudo apt-get install -y -qq gcc-4.9 g++-4.9 zlib1g-dev libbz2-dev libsnappy-dev libjemalloc-dev
+ - sudo apt-get install -y -qq gcc-4.9 g++-4.9 zlib1g-dev libbz2-dev libsnappy-dev
  - sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.9 50
  - sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.9 50
  - wget https://gflags.googlecode.com/files/libgflags0_2.0-1_amd64.deb