fix .travis.yml script exit code for VERSION >= v0.5 tests

[jrevels]

Right now, the test script looks like:

if (julia -e 'VERSION < v"0.5" && exit(1)'); then
    julia -e 'include(joinpath(JULIA_HOME, Base.DATAROOTDIR, "julia", "build_sysimg.jl")); build_sysimg(force=true)';
    julia -e 'Pkg.clone(pwd()); Pkg.build("ForwardDiff"); Pkg.test("ForwardDiff"; coverage=true)';
    julia -O3 -e 'include(joinpath(Pkg.dir("ForwardDiff"), "test/SIMDTest.jl"))';
else
    julia -e 'Pkg.clone(pwd()); Pkg.build("ForwardDiff"); Pkg.test("ForwardDiff"; coverage=true)';
fi

The returned error code in the first branch isn't technically correct, since it only reflects the outcome of the SIMD tests and not the whole test run. This can cause Travis to report success even if all the tests didn't actually succeed (see example here).