do not use jemalloc with clang on Travis CI builds

Summary:
The Travis CI builds use Ubuntu trusty, and the version of jemalloc that it
ships with is 4+ years old and does not work with clang+libstdc++.  clang
thinks that jemalloc's definitions of memalign and other functions conflict
with those provided by libstdc++.  This was fixed in upstream jemalloc in
2015 (jemalloc commit ae93d6bf3), but this fix is not available in Ubuntu
trusty.

This hasn't caused problems until now since the autoconf-based build of folly
did not use jemalloc unless arguments were supplied to configure explicitly
asking for jemalloc to be used.  The CMake-based build of folly uses jemalloc
automatically if it is available.

Reviewed By: phoad

Differential Revision: D10408985

fbshipit-source-id: 80afedcc079d1cd638863f6091ba322b708ccd85

Author: simpkins

.travis.yml

@@ -25,7 +25,6 @@ addons:
       - libgflags-dev
       - libgoogle-glog-dev
       - libiberty-dev
-      - libjemalloc-dev
       - liblz4-dev
       - liblzma-dev
       - libsnappy-dev
@@ -56,6 +55,7 @@ matrix:
           packages:
             - *common_deps
             - g++-5
+            - libjemalloc-dev
 
     - env: COMPILER_EVAL="CC=gcc-6 CXX=g++-6"
       addons:
@@ -65,6 +65,7 @@ matrix:
           packages:
             - *common_deps
             - g++-6
+            - libjemalloc-dev
 
 env:
   global: