travis, pls

.travis.yml

@@ -5,6 +5,8 @@ env:
     - BOOST_DIR=$HOME/boost_1_63_0
     - SLIC3R_STATIC=1
 script:
+    - if [ -z ${CXX7+x} ]; then export CXX=$CXX7; fi
+    - if [ -z ${CC7+x} ]; then export CC=$CC7; fi
     - bash package/linux/travis-setup.sh
     - cmake -DBOOST_ROOT=$BOOST_DIR src/
     - make
@@ -51,8 +53,8 @@ matrix:
         - os: linux
           env: 
             - WXVERSION=pkg
-            - CC=gcc-7
-            - CXX=g++-7
+            - CC7=gcc-7
+            - CXX7=g++-7
         - os: osx      
           env:
             - WXVERSION=pkg