travis: build zig with gcc to match what llvm was built with

from #llvm IRC:

<andrewrk> does llvm 7 as a .so have some kind of new initialization
requirement? I'm getting a segfault in llvm::DIBuilder::createFile
(with valid non-null parameters), when linking my frontend against
llvm-7.so but not when linking against llvm .a libraries

<d0k> we have an ABI bug in LLVM 7 when the .so is built with gcc
but your program is build with clang. I'm sorry for that.

ci/travis_linux_install

@@ -4,4 +4,4 @@ set -x
 
 sudo apt-get remove -y llvm-*
 sudo rm -rf /usr/local/*
-sudo apt-get install -y libxml2-dev clang-7 libclang-7-dev llvm-7 llvm-7-dev cmake s3cmd
+sudo apt-get install -y libxml2-dev libclang-7-dev llvm-7 llvm-7-dev cmake s3cmd gcc-7 g++-7

ci/travis_linux_script

@@ -3,8 +3,8 @@
 set -x
 set -e
 
-export CC=clang-7
-export CXX=clang++-7
+export CC=gcc-7
+export CXX=g++-7
 echo $PATH
 mkdir build
 cd build