For Travis CI - reduce parallel build jobs to 2 for MinGW static lib.
This is necessary because the last two CI builds have failed for MinGW static library build due to insufficient memory during linking phase.

Also:
- Increase the number of build artifacts being kept in the SourceForge.net.
- Fixes #380 by skipping Mach-O binary (universal library) for iOS 64-bit build. The Urho3D lib in the build artifact will only contain 64-bit arch, 32-bit arch and iphonesimulator are excluded for now to reduce the build time.
For Travis CI - switch CI build to use OS X build environment.