Don't use jemalloc with clang-based builds

Update the Travis CI configuration to stop installing libjemalloc-dev
for clang-based builds.  The version of libjemalloc that comes with
Ubuntu trusty is 4+ years old, and has bugs preventing it from working
when using clang with libstdc++.  (In particular jemalloc commit
ae93d6bf3 fixes these bugs, but is not available in the version of
jemalloc that ships with Ubuntu trusty.)

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: