.travis.yml: add 'dist: precise' for config that requires it

.travis.yml

@@ -23,6 +23,7 @@ matrix:
 # Performance test with GCC
     - os: linux
       compiler: g++
+      dist: precise
       env: OPJ_CI_ARCH=x86_64 OPJ_CI_BUILD_CONFIGURATION=Release OPJ_CI_INCLUDE_IF_DEPLOY=1 OPJ_CI_PERF_TESTS=1
 
 # Test compilation with AVX2
@@ -43,6 +44,7 @@ matrix:
 # Test multi-threading
     - os: linux
       compiler: g++
+      dist: precise
       env: OPJ_CI_ARCH=x86_64 OPJ_CI_BUILD_CONFIGURATION=Release OPJ_NUM_THREADS=2
 
 # Test 32-bit compilation
@@ -113,6 +115,7 @@ matrix:
     - os: linux
       compiler: g++-4.8
       env: OPJ_CI_CC=gcc-4.8 OPJ_CI_CXX=g++-4.8 OPJ_CI_ABI_CHECK=1
+      dist: precise
       addons:
         apt:
           sources: