Keep using gcc7 on Linux because gcc4.8 has bugs and mm doesn't build.

.travis.yml

@@ -4,6 +4,13 @@ matrix:
   include:
     - os: linux
       compiler: gcc
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-7
+      env: OS_NAME=Linux MATRIX_EVAL="CC=gcc-7 && CXX=g++-7"
     - os: osx
       compiler: clang
       env: OS_NAME=Darwin
@@ -15,7 +22,7 @@ before_install:
 script:
   - export VERSION=`echo "$(git tag -l --points-at HEAD)"`
   - mkdir build && cd build
-  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then cmake -DMM_VERSION="$VERSION" ..; fi
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then cmake -DMM_VERSION="$VERSION" -DCMAKE_C_COMPILER=/usr/bin/gcc-7 -DCMAKE_CXX_COMPILER=/usr/bin/g++-7 ..; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then cmake -DMM_VERSION="$VERSION" ..; fi
   - cmake --build . --target marketmaker-mainnet
   - cmake --build . --target marketmaker-testnet