Install g++-7 on before_install. Cache apt on travis build.

.travis.yml

@@ -4,12 +4,6 @@ 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
@@ -17,6 +11,7 @@ matrix:
       osx_image: xcode9.2
 
 before_install:
+  - sudo add-apt-repository ppa:ubuntu-toolchain-r/test && sudo apt-get update && sudo apt-get install -y g++-7
   - git submodule update --init --recursive
 
 script:
@@ -28,6 +23,7 @@ script:
   - cmake --build . --target marketmaker-testnet
 
 cache:
+  apt: true
   directories:
    - $HOME/.hunter