travis: Disable LLVM assertions on OSX

Our OSX builders are routinely and significantly over hour 2 hour "soft limit"
for testing PRs. I *think* that a big portion of this time comes from the fact
that LLVM and debug assertions are enabled. In an effort to speed up these
builders and reduce cycle time this commit disables LLVM assertions on OSX for
all builders.

My thinking is that we'll let this bake for a bit after merged to see what the
effect is on timing on Travis. If it doesn't actually help much we can turn them
back on, and if it doesn't help enough we can disable Rust debug assertions as
well.

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