Revert "Temporarily disable the clang build on travis-CI"

This reverts commit d1b799e5.

.travis.yml

 language: cpp
 
 compiler:
-  # - clang
+  - clang
   - gcc
 
 env:
@@ -13,8 +13,8 @@ env:
 
 matrix:
   exclude:
-    # - compiler: gcc
-    #   env: TRAVIS_BUILD_TYPE=Debug CCACHE_DIR=$HOME/.ccache_debug
+    - compiler: gcc
+      env: TRAVIS_BUILD_TYPE=Debug CCACHE_DIR=$HOME/.ccache_debug
 
 # use travis-ci docker based infrastructure
 sudo: false
@@ -28,12 +28,12 @@ addons:
   apt:
     sources:
     - kubuntu-backports
-    # - llvm-toolchain-precise-3.5
+    - llvm-toolchain-precise-3.5
     - ubuntu-toolchain-r-test
     packages:
     - autoconf
     - ccache
-    # - clang-3.5
+    - clang-3.5
     - cmake
     - g++-4.8
     - gdb