Build: attempt multi-compiler in Travis CI

.travis.yml

@@ -7,12 +7,14 @@ sudo: required
 dist: trusty
 language: cpp
 compiler:
-- gcc
+- gcc-5
+- clang-3.5
 addons:
   apt:
     packages:
     - gcc-5
     - g++-5
+    - clang-3.5
     - libssl-dev
     - libssl1.0.0
     - libminiupnpc-dev
@@ -24,7 +26,6 @@ before_install:
 - sudo add-apt-repository -y ppa:kojoley/boost
 - sudo apt-get -q update
 - sudo apt-get -y install libboost-{chrono,log,program-options,date-time,thread,system,filesystem,regex,test}1.58{-dev,.0}
-before_script: export CC=gcc-${GCC_VERSION} CXX=g++-${GCC_VERSION}
 script: make clean && make -j2 dependencies && make -j2 all-options && make -j2 tests && make doxygen && make install-resources
 after_success: ./monero-project/kovri/build/kovri-tests && ./monero-project/kovri/build/kovri-benchmarks
 notifications: