TravisCI: Rollback to using current Clang on Travis VMs

Change from Clang 3.9 to 3.5 (current version available).

3.9 was shown to take longer than 3.5 and having random segfaults here and there.

.travis.yml

@@ -9,16 +9,11 @@ git:
   depth: 1
 
 before_install:
-  - wget -O - http://apt.llvm.org/llvm-snapshot.gpg.key|sudo apt-key add -
-  - echo "yes" | sudo add-apt-repository 'deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-3.9 main'
-  - sudo apt-get -qq update
-  - sudo apt-get -qq install clang-3.9
   - sudo apt-get -qq install build-essential libtool make cmake cmake-data openssl
   - sudo apt-get -qq install libssl-dev libmysqlclient-dev libmysql++-dev libreadline6-dev zlib1g-dev libbz2-dev
   - sudo apt-get -qq install libboost1.55-dev libboost-thread1.55-dev libboost-filesystem1.55-dev libboost-system1.55-dev libboost-program-options1.55-dev libboost-iostreams1.55-dev libboost-regex1.55-dev
   - git config user.email "travis@build.bot" && git config user.name "Travis CI"
   - git tag -a -m "Travis build" init
-  - export CC=clang-3.9 CXX=clang++-3.9
 
 install:
   - mysql -uroot -e 'create database test_mysql;'