Reduce the number of if conditions in Travis config

.travis.yml

@@ -33,12 +33,8 @@ matrix:
   allow_failures:
     - env: ANALYZE="scan-build-3.6 --use-cc clang-3.6 --use-c++ clang++-3.6 "
 
-before_install:
- - if [ "${TRAVIS_OS_NAME}" = "linux" ]; then ./CI/before_install.linux.sh; fi
- - if [ "${TRAVIS_OS_NAME}" = "osx" ]; then ./CI/before_install.osx.sh; fi
-before_script:
- - if [ "${TRAVIS_OS_NAME}" = "linux" ]; then ./CI/before_script.linux.sh; fi
- - if [ "${TRAVIS_OS_NAME}" = "osx" ]; then ./CI/before_script.osx.sh; fi
+before_install: ./CI/before_install.${TRAVIS_OS_NAME}.sh
+before_script: ./CI/before_script.${TRAVIS_OS_NAME}.sh
 script:
  - cd ./build
  - if [ "$COVERITY_SCAN_BRANCH" != 1 ]; then ${ANALYZE}make -j3; fi