Run tests with different gcc versions

.travis.yml

@@ -1,18 +1,19 @@
+sudo: required
 language: rust
 rust:
   - nightly
 script:
+  - sudo -E apt-add-repository -y "ppa:ubuntu-toolchain-r/test"
+  - sudo -E apt-get -yq update &>> ~/apt-get-update.log
+  - sudo -E apt-get -yq --no-install-suggests --no-install-recommends --force-yes install gcc-$COMPILER_VER g++-$COMPILER_VER
   - mkdir -p gcc-symlinks
-  - ln -s /usr/bin/g++-6 gcc-symlinks/g++
-  - ln -s /usr/bin/gcc-6 gcc-symlinks/gcc
-  - ln -s /usr/bin/gcov-6 gcc-symlinks/gcov
+  - ln -s /usr/bin/g++-$COMPILER_VER gcc-symlinks/g++
+  - ln -s /usr/bin/gcc-$COMPILER_VER gcc-symlinks/gcc
+  - ln -s /usr/bin/gcov-$COMPILER_VER gcc-symlinks/gcov
   - export PATH=$PWD/gcc-symlinks:$PATH
   - cargo build --verbose
   - cargo test -- --nocapture
-addons:
-  apt:
-    sources:
-    - ubuntu-toolchain-r-test
-    packages:
-    - gcc-6
-    - g++-6
+env:
+  - COMPILER_VER=4.9
+  - COMPILER_VER=5
+  - COMPILER_VER=6