Fix Travis-ci builds (#950)

Line "- /usr/local/bin:$PATH" is misformatted.
It must be something like "- PATH=/usr/local/bin:$PATH".

It seems something changed in tarvis-ci month ago and now this leads to:

Setting environment variables from .travis.yml
$ export PATH=

Defailt PATH is /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin
so already containts /usr/local/bin.


Image "xcode8.3" contains macOS 10.12 (sierra) which has no bottles
with precompiled gcc-7 in homebrew storage.

Image "xcode9.4" is a current default with macOS 10.13 (high_sierra).

Link: https://docs.travis-ci.com/user/reference/osx/
Link: https://formulae.brew.sh/formula/gcc@7

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>

.travis.yml

@@ -2,10 +2,6 @@ sudo: required
 dist: trusty
 language: cpp
 
-env:
-  global:
-    - /usr/local/bin:$PATH
-
 matrix:
   include:
     - compiler: gcc
@@ -169,7 +165,7 @@ matrix:
         - BUILD_32_BITS=ON
         - EXTRA_FLAGS="-m32"
     - os: osx
-      osx_image: xcode8.3
+      osx_image: xcode9.4
       compiler: gcc
       env:
         - COMPILER=g++-7 C_COMPILER=gcc-7  BUILD_TYPE=Debug