Explicitly set compiler in the matrix
Explicitly set compiler in the matrix

When using compiler: instruction to specify both gcc and g++, it sets CC
to ["gcc", "g++"] and causing make to look for an executable "[gcc,".

Didn't see this issue in the previous buildings, could be an issue when
used with matrix.include

Here's what's in the travis build log:

Setting environment variables from .travis.yml
$ export OS_ID=0.2.40
$ export CC=["gcc", "g++"]
/Users/travis/build.sh: line 62: export: `g++]': not a valid identifier

codebybrett/ren-c,draegtun/ren-c,rgchris/ren-c,kealist/ren-c,hostilefork/rebol,hostilefork/rebol,giuliolunati/ren-c,codebybrett/ren-c,kealist/ren-c,codebybrett/ren-c,giuliolunati/ren-c,hostilefork/rebol,kealist/ren-c,rgchris/ren-c,codebybrett/ren-c,hostilefork/rebol,giuliolunati/ren-c,giuliolunati/ren-c,rgchris/ren-c,codebybrett/ren-c,rgchris/ren-c,hostilefork/rebol,kealist/ren-c,draegtun/ren-c,kealist/ren-c,draegtun/ren-c,giuliolunati/ren-c,kealist/ren-c,codebybrett/ren-c,hostilefork/rebol,draegtun/ren-c,draegtun/ren-c,rgchris/ren-c,draegtun/ren-c