SourceMod doesn't support Source 1 Dota 2 anymore and neither Source 2. Speed up CI by only cloning the hl2sdks that are tested.
This adds a `-s sdk1,sdk2` parameter to the checkout-deps.sh script similar to the `-SDKs` option of the powershell script to select specific sdks to update instead of all.
Our official builds use clang-3.4 (for macOS) and clang-3.8 for Linux.
Linux uses libstdc++-4.9. Make sure these two compilers are being tested
and that libstdc++-4.9 is being used for STL.
Add a macOS builder to get coverage there. This will use a newer
clang than we actually use, but as opposed to the linux builder will
test the platform-specific bits.
Finally, use the latest GCC and clang versions from a bionic image, so
we have some coverage of a popular distribution.
I doubt we're interested in the filename of every single file
contained in the mysql archive when it's decompressed.
It bloats up the travis build log.
