[Mod] use update-alternatives again

.travis.yml

@@ -15,10 +15,6 @@ services:
   - mysql
   - postgresql
 
-before_install:
-  - echo eval "${MATRIX_EVAL}"
-  - eval "${MATRIX_EVAL}"
-
 before_script:
   - psql -d postgresql://postgres:${VNPY_TEST_POSTGRESQL_PASSWORD}@localhost -c "create database vnpy;"
   - mysql -u root --password=${VNPY_TEST_MYSQL_PASSWORD} -e 'CREATE DATABASE vnpy;'
@@ -45,8 +41,10 @@ matrix:
             - ubuntu-toolchain-r-test
           packages:
             - g++-8
-      env:
-        - MATRIX_EVAL="CC=gcc-8 && CXX=g++-8"
+      before_install:
+        - sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-7 90
+        - sudo update-alternatives --install /usr/bin/c++ c++ /usr/bin/g++-7 90
+        - sudo update-alternatives --install /usr/bin/gcc cc /usr/bin/gcc-7 90
       install:
         # update pip & setuptools
         - python -m pip install --upgrade pip wheel setuptools
@@ -54,15 +52,17 @@ matrix:
         - pip install https://pip.vnpy.com/colletion/ibapi-9.75.1-001-py3-none-any.whl
         - bash ./install.sh
 
-    - name: "sdist install under Ubuntu: gcc-8"
+    - name: "sdist install under Ubuntu: gcc-7"
       addons:
         apt:
           sources:
             - ubuntu-toolchain-r-test
           packages:
             - g++-7
-      env:
-        - MATRIX_EVAL="CC=gcc-7 && CXX=g++-7"
+      before_install:
+        - sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-7 90
+        - sudo update-alternatives --install /usr/bin/c++ c++ /usr/bin/g++-7 90
+        - sudo update-alternatives --install /usr/bin/gcc cc /usr/bin/gcc-7 90
       install:
         # Linux install script
         - python -m pip install --upgrade pip wheel setuptools