Travis: Use gcov that matches the used compiler.

This fixes gcov/coveralls warnings like the following:

    Segmentation fault (core dumped)
    charset.c.gcno:version '501*', prefer '406*'
    Out of memory allocating 33061786568 bytes after a total of 2522648 bytes

http://stackoverflow.com/a/14676272/249642

.ci/after_success.sh

@@ -7,4 +7,4 @@ if [[ -n "${CI_TARGET}" ]]; then
   exit
 fi
 
-coveralls --encoding iso-8859-1 || echo 'coveralls upload failed.'
+coveralls --gcov "$(which "${GCOV}")" --encoding iso-8859-1 || echo 'coveralls upload failed.'

.travis.yml

@@ -64,26 +64,29 @@ matrix:
       env: CI_TARGET=clint
     - os: linux
       compiler: gcc-5
+      env: GCOV=gcov-5
     - os: linux
-      env: BUILD_32BIT=ON
       # Travis creates a cache per compiler.
       # Set a different value here to store 32-bit
       # dependencies in a separate cache.
       compiler: gcc-5 -m32
+      env: GCOV=gcov-5 BUILD_32BIT=ON
     - os: linux
-      env: CLANG_SANITIZER=ASAN_UBSAN
       compiler: clang-3.6
+      env: GCOV=llvm-cov-3.6 CLANG_SANITIZER=ASAN_UBSAN
     - os: linux
-      env: CLANG_SANITIZER=MSAN
       compiler: clang-3.6
+      env: GCOV=llvm-cov-3.6 CLANG_SANITIZER=MSAN
     # FIXME (tarruda): Uncomment when TSan tests don't hang anymore.
     #- os: linux
-    #  env: CLANG_SANITIZER=TSAN
     #  compiler: clang-3.6
+    #  env: GCOV=llvm-cov-3.6 CLANG_SANITIZER=TSAN
     - os: osx
       compiler: clang
+      env: GCOV=gcov
     - os: osx
       compiler: gcc-4.9
+      env: GCOV=gcov-4.9
     - os: linux
       env: BUILD_MINGW=ON
   fast_finish: true