Not sure why we switched from homebrew to scons-local anymore,
but pip should work just fine and is recommended by upstream.
Downloads of scons-local from SourceForge were quite iffy, so
this should help avoid CI errors due to not having downloaded
SCons successfully.
Setting Python's user binary PATH seems necessary on Travis/OSX
as per travis-ci/travis-ci#5030 (confirmed that 'scons' is not
in PATH without it).
As of clang-format 6.0.1, putting the `/* clang-format off */` hint
around our "invalid" `[vertex]` and `[shader]` statements isn't enough
to prevent a bogus indent of the next comments and first valid statement,
so we need to enclose that first valid statement in the unformatted chunk.
Commits or PRs that do not respect the clang-format style that we
enforce will fail the test.
Adding ubuntu-toolchain-r-test as its libstdc++ is necessary to install
libllvm-3.9.
