Fix Android 32-bit build

[jdonald]

Here's the fix that @yeswalrus recommended, using stringstream.

I've clarified the preprocessor conditions so we have one less issue when eventually upgrading from gnustl to libc++.

Travis doesn't build Android yet, so I verified both 32-bit and 64-bit builds locally.

• Fix Travis clang build first
• Check MSVC
• Fixes the issue introduced in PR Config/stl marshal #1003

[yeswalrus]

@hham I checked MSVC, as soon as you verify this fixes the android 32 bit builds it should be good to go.

[hham]

Oh good. I was building on my Windows laptop.

[jdonald]

Thanks guys!