set travis open file descriptor limit

Summary:
max is 8192 on mac and this value seems to prevent the tests from
running out of open fds while running

Test Plan:
committed to separate branch to trigger test runs:
https://travis-ci.org/facebook/rocksdb/builds/150817199

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D61767

.travis.yml

@@ -33,8 +33,8 @@ before_script:
   - export CPLUS_INCLUDE_PATH="$CPLUS_INCLUDE_PATH:/tmp/gflags-2.1.2/include"
   - if [[ "${TRAVIS_OS_NAME}" == 'linux' ]]; then CXX=clang++-3.6; fi
   - if [[ "${TRAVIS_OS_NAME}" == 'osx' ]]; then brew install gflags snappy; fi
-  # Limit the maximum number of open file descriptors to 2000
-  - ulimit -n 65535 || true
+  # Limit the maximum number of open file descriptors to 8192
+  - ulimit -n 8192 || true
 
 script:
   - if [[ "${JOB_NAME}" == 'unittests' ]]; then OPT=-DTRAVIS V=1 make -j4 check_some; fi