geckodriver: ci: force gcc compiler for i686-unknown-linux-musl

Travis at some point changed the default compiler in their images to be
clang.  Cross-compiling Rust code with clang is not possible quite yet,
so we force gcc to be used.

Fixes: https://github.com/mozilla/geckodriver/issues/495
Source-Repo: https://github.com/mozilla/geckodriver
Source-Revision: 043806820230f720c253d3d305dc15747d994b05

committer: jgraham <james@hoppipolla.co.uk>

--HG--
extra : rebase_source : 5b3a96f126a2b657e7659450489a99451ea4103b

testing/geckodriver/.travis.yml

@@ -35,6 +35,7 @@ matrix:
         - TARGET=i686-unknown-linux-musl
         - NAME=linux32
         - EXT=tar.gz
+        - CC=gcc
       dist: trusty
       sudo: required
       addons: